Shakyamuni Buddha is the Father and Nichiren Daishonin is the messenger.

The Daishonin is Bodhisattva Jogyo, disciple of the Eternal Original Buddha Shakyamuni.

"Since the Buddha of the Juryo chapter is revealed as the eternal Buddha, it follows that the great Bodhisattvas such as Monju and Miroku, and the great bodhisattvas from other realms are in fact disciples of Shakyamuni Buddha. If, among all the numerous sutras, this Juryo chapter should be lacking, it would be as though there were no sun and moon in the sky, no supreme ruler in the nation, no gems in the mountains and rivers, and no spirit in man. "(MW, V2, pg. 150)

"Since Sakyamuni Buddha is eternal and all other Buddhas in the universe are His manifestations, then those great bodhisattvas converted by manifested Buddhas are also disciples of Lord Sakyamuni Buddha. If the "Life Span of the Buddha" chapter had not been expounded, it would be like the sky without the sun and moon, a country without a king, mountains and rivers without gems, or a man without a soul." (Noppa, pg. 176, Kaimoku Sho)

"Thus it was revealed that Shakyamuni had long been the Buddha since the eternal past, and it became clear that various Buddhas in other worlds were all manifestations of Shakyamuni Buddha..[ ] now, however, as Shakyamuni was proved to be the Eternal Buddha, those Buddhas in the Flower Garland Sutra, or Buddhas in the Hodo, Hannya, or Great Sun Buddha sutras all became subordinates of Shakyamuni Buddha." (Kaimoku Sho, p. 174, translated by Kyotsu Hori, 1987)

BTW, here's the same quote, as translated by NSIC. Please note the differences:

"When Shakyamuni Buddha revealed that he had gained enlightenment in the far distant past and had since then been constantly in the world, it became apparent that all the other Buddhas were emanations of Shakyamuni....but now it becomes apparent that Vairocana Buddha of the Kegon Sutra and various Buddhas of the Hodo, Hannya and Dainichi sutras are in fact all followers of Shakyamuni Buddha." (MW V.2, p.149)

"Since Shakyamuni Buddha is eternal and all other Buddhas in the universe are His manifestations........" (Kaimoku Sho, translated by Kyotsu Hori, p.176).

"But now that it has become apparent that Shakyamuni Buddha attained enlightenment countless aeons ago,.......etc. (MW V.2, p. 151)

Not only is there no mention of Nichiren as "True" Buddha in the Kaimoku Sho but in the Kanjin Honzon Sho, translated by Kyotsu Hori, nowhere, does Nichiren mention that he is the "True" Buddha:

"...Shakyamuni Buddha, within our minds, is an ancient Buddha without beginning, manifesting Himself in three bodies, he attained Buddhahood in the eternal past......(ibid. P.94)

"Shakyamuni Buddha, the Lord-preacher of this pure land, has never died in the past, nor will he be born in the future. He exists forever, throughout the past, present and future." (P.100)

"Many wooden statues and portraits were made of Shakyamuni Buddha as He preached Hinayana and quasi-Mahayana sutras, but statues and portraits of the Eternal Shakyamuni Buddha revealed in the Juryo Chapter of the Lotus Sutra were never made. Now, in the beginning of Mappo, is it not the time that such statues and portraits are made?" (P.104)

The Hoon Sho (translated by Taikyo Yajima) states:

"All the people in Japan as well as the rest of the whole world should revere Lord Buddha Shakyamuni revealed in the essential section (honmon) of the Lotus Sutra as the object of worship (honzon)." (P. 198)

The Lotus Sutra itself states:

"For the sake of curing his children gone mad,

Though in reality he exists, yet he says he dies,

And there are none who can declare it is a vain lie;

I likewise am the Father of the World,

The One who saves from the various sufferings and travails.

Because the unenlightened worldlings are wrongheaded,

Though in reality I exist, I say I am extinguished;

because by seeing me constantly,

They would produce an arrogant and licentious mind,

Self abandoned, attached to the five desires,

And would fall among the Evil ways of rebirth.

I ever know the masses of beings,

Proceeding or not proceeeding on the Way;

According to what will save them,

I preach for them all sorts of Dharmas.

I Myself ever form this thought:

'By what shall I cause the masses of beings

To be able to enter the supreme Way

And rapidly achieve Buddhahood."(Lotus Sutra, Chapter 16)

Now we can begin to examine how Nichiren viewed himself.

"...the Bodhisattvas (who sprang from underneath the ground), as described in the 15th chapter, are followers of the Original Buddha Shakaymuni who resides within our minds."(p.94)

" The object of veneration (Honzon) at the scene of this transmission of Namu Myoho Renge Kyo from the Eternal Buddha to his original disciples is suspended in the sky above the Eternal Buddha Sakyamuni's Saha-world. 

The Lotus Sutra, Chapter 16 states, "Then he sent a messenger back to his children, telling them that their father has passed away." In the Kanjin Honzon Sho, Nichiren explains the phrase: ''send a messenger back", as those bodhisattvas who were called out from underground.

The SGI and Nichiren Shoshu argue that Nichiren Daishonin states in the Opening of the Eyes that, "I will be the Pillar of Japan; I will be the Eyes of Japan; I will be the Great Vessel of Japan, proving that Nichiren possesses the Three Virtues of the Original Buddha. But this is a mistake for the following reasons:

1). It can be contrasted to the following passage in the Kaimoku Sho where Nichiren states that Shakyamuni possesses the Three Virtues for "all beings" : "One should know that the Enlightened One, the Buddha, is a Great Teacher for all living beings, a Great Eye for them, a Great Pillar, a Great Helmsman, a Great Field of Plenty." (MW vol 2, Opening of the Eyes, pg 77)

2). The above passage of the Lotus Sutra states: "I am the Father of the world" [not just the little island nation of Japan].

3). Nichiren in many other Gosho explains that Shakyamuni is the parent, teacher and sovereign of the entire world (Admonitions Against Slander, Letter to Nikke, Letter to Nichimyo Shonin, The Learned Doctor Shan-wu-wei, Letter to Myomitsu Shonin, Rebuking Slander of the Law, Letter to Ichinosawa Nyudo, On Prayer and others).

Now we can move on to some of the passages in both the Lotus Sutra and the writings of Nichiren revealing the Daishonin's true identity:

"I Nichiren, humble person though I am, have received Lord Shakyamuni's royal command and come to this country of Japan." (MW vol 5, The Pure and Far Reaching Voice, pg 143)

The Original Eternal Buddha has never received the royal command of anyone, he invokes them.

"Will the Bodhsattva Visista-caritra (Jogyo) appear in these days of the Latter Law to open wide the gateway of the Truth, or will he not appear? The Scripture tells us so; yet will it surely happen? Will the Bodhisattva appear or not? At any rate, I, Nichiren have now accomplished the pioneerwork." (Selection of the Time).

"For one aeon, two aeons, countless aeons I devoted myself to the practices of a bodhisattva, until I had almost reached the state where I couldn't never fail to attain Buddhahood. And yet I was dragged down by the powerful and overwhelming influence of evil, and I never attained Buddhahood. I do not know whether I belong to the third group of those who heard the preaching of the sons of Daitsu Buddha and yet failed to attain enlightenment even when one son was reborn in the world as Shakyamuni Buddha, or whether I faltered and fell away from the teachings which I heard long before Daitsu Buddha, at gohyaku jintengo and thus have been reborn in this form. (MW,V2, pg.112)

Moreover, I am a poor monk without social status. While having traversed the Six Realms in my past lives, I have sometimes been born a great king in the human or celestial realm, making everyone obey me just as a strong wind sways the twigs of small trees. Still, I could not attain Buddhahood then. At other times, I must have gone up step by step the way a great bodhisattva according to Hinayana and Mahayana sutras. Having practiced the way of a bodhisattva for as long as one, two and in numerous kalpa, I was about to reach the state of non-retrogression. However, a powerful, evil karma prevented me from obtaining Buddhahood.

According to the "Parable of a Magic City" chapter of the Lotus Sutra there were three categories of people who had an opportunity to listen to the sutra and sow the seed of Buddhahood during the time of Daitsuchisho Buddha in the past. They were all to attain Buddhahood eventually with the third and last group being guaranteed of being future Buddhas upon listening to Shakyamuni Buddha preach the Lotus Sutra. I wonder whether or not I was excluded from this third category of people. Or, am I one of those who, while having listened to the Lotus Sutra and sowed the seed of Buddhahood 500 dust-particle kalpa (gohyaku-jindengo) in the past, have kept falling back till today without eventually getting it? .. (Noppa, Kaimoku Sho, page 92-94)

"I Nichiren, received the Buddha's edict and was born in this land . ." (On Practicing According to the Preaching" (Nyosetsu shugyo sho STN, v. 1, 733 1, 2), 

From Lotus Sutra Chapter 21:

Because, after the Buddha's extinction,

It is possible to possess this sutra,

The buddhas all rejoice

And show infinite powers divine.

Now that this sutra is entailed

To him who keeps it, let praise,

Through kalpas infinite,

Be inexhaustible.

The merits of this man

Shall be boundless and without end

As space in every direction,

Which cannot find a limit.

He who can keep this sutra

Is one who already beholds me

And also the Buddha Abundant Treasures,

And all buddhas emanated [from me],

And sees besides the bodhisattvas

Whom I have instructed until now.

He who can keep this sutra

Will cause me and the [buddhas] emanated from me,

And the Buddha Abundant Treasures in nirvana,

All of us entirely to rejoice;

And the buddhas now in the universe,

And those of the past and the future,

He shall also see and serve

And cause to rejoice.

The mysterious laws that have been attained

By the buddhas each on his wisdom throne,

He who can keep this sutra

Must surely gain ere long.

He who can keep this sutra

Shall the meaning of the laws,

With their terms and expressions,

Delightedly expound without end,

Like the wind in the sky,

Which never has impediment.

After the Tathagata is extinct [such a one],

Knowing [this] sutra that the Buddha has taught,

[Together with] its reasoning and process,

Shall expound it according to its true meaning.

Just as the light of the sun and moon

Can dispel the darkness,

So this man, working in the world,

Can disperse the gloom of the living

And cause numberless bodhisattvas

Finally to abide in the One-vehicle.

Therefore he who has wisdom,

Hearing the benefits of this merit,

After I am extinct,

Should receive and keep this sutra.

This man shall in the Way of the Buddha

Be fixed and have no doubts."

The Daishonin says:

" I Nichiren may be a fool, but, having declared myself to be the messenger of Shakyamuni Buddha and the Votary of the Lotus Sutra, it is nothing short  of amazing that my words go unheeded"-- Letter To Ichinosawa Nyudo,

"Until the envoy of the Buddha actually appeared to expound the Lotus Sutra... However, now that I am spreading the Lotus Sutra as the Buddha'senvoy , everyone -- from ruler to the lowliest subject-- has become a slanderer."---Letter to Niike).

and

I, Nichiren, may not be an envoy sent by the Buddha, but my appearance in this world coincides with the age of the Latter Day. Moreover, quite unexpectedly, I came to realize this teaching, which I now expound to prepare the way for a sage. (Letter to Misawa)

Graham Lamont writes:

In 1277 (the third year of the Kenji era) Nichiren Daishonin wrote a deposition on behalf of Shijo Kingo Yorimoto (the autograph no longer exists but TWO versions copied by Nikko still exist at the Omosu Hommonji (STN v.2, 1346, 1360 notes):

1) an unrevised version compiled in 1278 (the first year of Koan) with the title "On the Dialogue with Ryuzo" or RYUZO MONDO SHO.

2) a "revised version" from the fifth year of Showa (1316) when Nikko was seventy-one years old. Since Nikko's colophon says that he copied this from a "revised version" (saiji hon) it is probable that Nichiren Daishonin rewrote some of the passages some time after 1278.

It is instructive that Nikko would copy this later in his life as a message to his disciples that the revised version is definitive.

Let's look at the correct faith for a disciple of Nichiren:

"I believe it is preached in the HOKEKYO that His Reverence Nichiren Daishonin is the BODHISATTVA JOGYO, the messenger the Tathagata Shakya, Lord of the Three Worlds, Father and Mother of All Beings.“ (STN v.2 1358 and note 1). The capitalized part, the words THE BODHISATTVA JOGYO are in the revised edition.

Nichiren Daishonin himself went to the trouble of adding thewords "the Bodhisattva Jogyo" and even if Nikko, after the death of Nichiren, added those words, here and in the other revised sections, it shows that Nikko regarded Nichiren Daishonin as the bodhisattva Jogyo.

This is proof of how Nichiren Daishonin or, at the very least Nikko, saw the Daishonin's true identity. This identity, which he sometimes hid, was that of the Bodhisattva Jogyo not of the Buddha.

Further evidence is found in the Gosho, The Plots of the elder of Gokurakuji,", the account of the plot of Ryokanbo Nisho, against Nichiren Daishonin. The unrevised account refers to the attempt to kill Nichiren and to the obstacles to him and his disciples on Sado but says nothing about Nichiren Daishonin identity. However, in the REVISED EDITION of Nikko,  though not in Nichiren's hand, it states:

"Nichiren Daishonin, if it is as it is preached in the Sutra, is the Sage who is the Messenger of the Tathagata Who Attained in the Distant Past (kujo nyorai no onzukai), the Descent (suijaku) of the Bodhisattva Jogyo, the Practicer (gyoja) of the Original Doctrine (or Aspect - Hommon) of the Hokke, the Great Guiding Teacher of the fifth five hundred years" (STN v.2, 1352)

Why would Nichiren or Nikko include these words which clearly illustrate that Nichiren Daishonin is the emissary from the Eternal Buddha and the manifestation of Bodhisattva Jogyo, unless they were attempting to show that he actually is the Bodhisattva Jogyo sent by the Buddha Shakyamuni?

In the Minobu period letters, nowhere did Nichiren Daishonin feel himself to be the Buddha, although he is certain of his attainment of Buddhahood. Rather, as he had said in the treatise "On Practicing According to the Preaching" (Nyosetsu shugyo sho STN, v. 1, 733 1, 2), "I Nichiren, received the Buddha’s Proclamation and was born in this land." He also says he has engaged in his crusade of shakubuku "because theDecree of the Dharma King is difficult to defy." (ibid) 

(Note that this Nyosetsu Shugyo Sho survives only in the manuscript of Nikko's disciple Tayu Nichizon (1265-1345), copied in 1297 (the fifth year of Einin).)

Now we can look at what Nikko's has to say:

"The Order of Spreading the Sutra in the Three Ages (Sanji gukyo shidai)(Nichirenshu shugku zensho, II, 53) specifically says:

"The Temple of the Hommon

The disciple of Entrustment

The Bodhisattva Jogyo Nichiren Daishonin."

It is clear that in the Original Doctrine (Hommon), the correct doctrine Nichiren Daishonin is considered the Manifestation of Jogyo, the Great Bodhisattva Converted by the Original Buddha, the disciple clearly delegated with the substance of the Sutra in the "Chapter of the Divine Powers of the Tathagatas" 21. This same work of Nikko refers to the Buddha Shakya Who Attained in the Past (Kujo no Shaka Butsu) the Supreme Enlightenment as the Object of Worship.

There is also proof to be found on a particular Gohonzon from 1274 (currently kept at Myohonji at Hota.) It is No. 16 in the Catalogue of the Gohonzon Collection (Yamanaka Kihachi, Nichiren Daishonin Shinseki no Sekai, p 323 and type B (II) 7 no. 13 in Nichiren Daishonin Mandara zushu, pp. 58-59) On it, in what is considered a "lauditary text", Nichiren writes:

"Since the entrance into Extinction of the Great Enlightened World Honored One there have passed in succession more than two thousand two hundred and twenty years. Even so among the Three Countries of India, Han (China) and Japan, there has not yet been this GREAT OBJECT OF WORSHIP (Dai Honzon). Either they have known but not yet spread it or they have not known it. Our Compassionate Father, by means of the Buddha Wisdom, has hidden and retained it, leaving it for the Latter Age. At the time of the last five hundred years, the Bodhisattva Jogyo comes forth in the world and for the first time spreads and proclaims it."