Pharos

Pharos is an important figure in Persona 3, appearing as a young child, he is first seen upon the arrival of the Protagonist in the dorm, where he gives the protagonist a contract, prompting him to sign his name. It is later revealed that the contract that Pharos had the protagonist sign allows the protagonist to access the Velvet Room, with his reasons and purpose unknown. He is shown to have a certain understanding towards the Shadows, and upon the Protagonist's first attempt of summoning his Persona, Pharos can be seen smiling in front of the protagonist, encouraging him to fire the Evoker.
Pharos is shown to be residing within the protagonist's psyche, where he shows up only within the period of the Dark Hour. It should also be noted that Pharos can only be heard and seen by the protagonist. Pharos constantly speaks of "the Fall", and would often reveal vague hints of the Dark Hour's nature, along with the "ordeals" every full moon. At one point Pharos would request the protagonist to be his friend, allowing the protagonist to forge the Death social link.
As S.E.E.S destroys the Shadows that appear during the Full Moon, Pharos slowly regains parts of his memories, along with his role. It is not until the destruction of the final Shadow, that Pharos' memory is fully restored and he is reminded of his role. He shows up the next morning, where he bids farewell to the protagonist, saying that it is time for their parting and that he would treasure the conversations and memories they've created.
Pharos' nature is later revealed. In the year 1999, when the experiment utilizing Shadows being conducted by the Kirijo Group was interrupted by Eiichiro Takeba, Death was forcefully ripped apart from his components, and escapes towards the Moonlight Bridge. Once there Aigis, the last remaining anti-Shadow weapon that survived Death's rampage, later engages the Shadow. However, even in an incomplete state, Death's potential was far greater than Aigis anticipated. In an act of desperation, Aigis sealed Death in a young boy's mind, using it as a vessel to keep Death from re-bonding with his counterparts. The young boy, who lost his parents to a 'car accident' caused by Aigis and Death's battle is the protagonist.
Inside the protagonist, Death slowly assumed a human form, as well as sub-consciously bringing the Protagonist to Port Island, the first place where Death was born. With the "emotions" and "feelings" the protagonist bestowed towards Pharos, he would later assume to form of an individual person, Ryoji Mochizuki after leaving the protagonist's psyche.