The Peacemouse is not seen but his voice can be heard however.
Not much is known since he never appears in-game.
He only starts to lure the suits to you by saying quotes which lure them near.
His voice is heard again and he remarks that he was an imposter looking for ways to get the suits to charge. He also mentions the terror that in the next game, he will return. However, when the screen cuts to black he says "RUN" in a demonic voice, then the ending ends.
"He was here..."
"I hid there....."
"He will die....."
"He hears us call to the dead....."
"One of them is near..... good luck...."