Ryouji Mochizuki

Spoiler, click to view

His true identity is "Nyx Avatar" as well as the Appriser and harbringer of "Death" and Nyx. He is also the older version of Pharos.

Originally, Ryoji shows up as a new transfer student, and becomes fast friends with Junpei and the rest of the group. His cheerful and reckless dating attitude is also subject of much talk, even going as far as asking Yukari as well as Mitsuru on a date upon first sight. Aigis remained extremely cautious of him, however, constantly referring to him as "dangerous."

Spoiler, click to view

The fun doesn't last long though, as a battle with Aigis on Moonlight Bridge finally awakens his memories of his true purpose. When the group gets there, they find a damaged Aigis, and an unconscious Ryoji. After he recovers, he tells them everything he knew, and expressed that he is tormented by the fact of who and what he is more than anyone. He gives the group a choice: they can kill him on the spot, and the Dark Hour will vanish, and they will forget everything, and return to their normal lives, or they can let him live. Either way, according to him, it is inevitable that the world will end at the hands of Nyx; only by choosing to kill them, will their world end in a somewhat "painless" fashion. Depending on the choice the player makes, they can either kill or let Ryoji live, each resulting in a different ending. During the discussion with the main character, he shows that he is also Thanatos, the Ultimate Persona of the Death Arcana.