Re: RJ's Stream - Undertale (I Am Arjay, I am Asriel... Frisk)
Frisk is Arjay's Literal Split Personality, with part of Asriel mixed in. Which explains why, in a Pacifist Run, you can achieve a True End while still being cruel, taunting and making fun of enemies or insulting and lying to your friends. Things Asriel would never do, but Arjay would out of cruelty. (As Children, Arjay poisoned Asgore with the yellow flower pollen as a prank and laughed while Asriel helped, but still didn't think it was very funny.)
The opening says that the child falling into Mt. Ebott takes place in 201x. There's an "old" calendar in Toriel's drawer labelled 201x. That's Arjay in the opening, not Frisk.
Arjay killed themselves via Yellow Flowers to give their Soul to Asriel, so they could pierce the barrier and kill humans. Asriel wanted to free the Monsters, Arjay wanted to kill humans, but Asriel couldn't go through with it, and the fusion fled the humans, dying. Asriel's dust started to fill up the yellow flower fields. Arjay's body was originally kept in a special coffin that kept his Soul intact, but his body stayed dead. Eventually, Toriel took Arjay's Body to the Ruins and buried it. Some stray seeds of the Yellow Flowers were buried also.
Arjay's body did not decay; something Monsters didn't realize was significant, because Monsters turn to dust immediately upon death; they assumed the fact that human bodies didn't mean they stay intact forever. The reason the body did not decay was because having been fused with Asriel, and having the yellow flowers infused with Asriel's dust by his body; there was a very small connection of two Boss Monsters' Souls. (Not enough to visibly age Toriel and Asgore, but it was there.) In time, it was enough for "Frisk" to grow his own soul, a hybrid with benefits of both human and monster; and he materializes above Arjay's grave, still wearing the bandage from the Fallen's coffin. (Same body).
With Frisk born with the combination of Arjay and Asriel for a soul, Alphys attempts to perform her experiments to revive Asriel by injecting Determination (presumably from Arjay's soul) into a flower that's been scattered with the dust of Asriel's dead body. Asriel comes back to life as a flower, but only has Determination, he still lacks a soul. Despite the Determination coming from Arjay or not, it carries no good or ill will. Instead Asriel slowly goes insane from his inability to feel anything and being unable to love his parents. The real Asriel exists inside Frisk as the combined souls of him and Arjay, and the events of Undertale are the conflict between the two souls coming to their conclusion.
In this case, Asriel won. Both his soul, and his empty shell, convinced Arjay to let the world be happy.