"I am thou..." is a common quote of the signature Personas of the protagonists of the Persona series when they are being summoned the first time. The complete quote varies a bit throughout the series, but the general meaning is almost the same. Archaic grammar is deliberately used here to denote the figure that the Persona is based on originated from ancient mythology.
Similar words may be spoken by Philemon depending on the actions the player takes at the end of Persona 2: Innocent Sin, referring to the fact that Philemon, as one personification of the Collective Unconscious, lives within the protagonists just as their Personas do. In Persona 5, not only the initial Personas of the playable members, all Shadows speak the same quote when they are reminded of coming from the sea of human souls after a successful negotiation.
"I am thou... Thou art I... From the sea of thy soul, I come..."
(Romaji: Ware wa nanji… nanji wa ware… Ware wa nanji no kokoro no umi yori izuru-mono…)"
After this part, one more verse follows by briefing the identity of the figure in the originating legend. Some examples are listed below.
"I shall lend you my strength..."
"The bearer of the inferno that burns away impurity, Vulcanus!"
"Thy heart that beats for thy friends lights mine holy flame... I am thou... Thou art I... I am Vulcanus, awakened to the ancient fire..."
"Ruler of the blazing sun and blue skies, Apollo..."
"I am the brilliant mother, Maia..."
"From the very moment of my emergence, I have been a guiding light shed to illuminate thy path...I am the original god...Izanagi-no-Okami."
"I am the one who dwells in the rift between human consciousness and unconsciousness. I am you. You are me... I am always within you, watching over you. Farewell..."
"Call upon my name, and release thy rage! Show the strength of thy will to ascertain all on thine own, though thou be chained to Hell itself! I am the pillager of twilight, Arsene!"