Never Leave Steve is an imaginary friend who is mentioned in "Squeeze the Day" who supposedly never leaves his room and is never shown throughout the entirety of the series.
History[]
Bloo enters Never Leave Steve's room to tell him about how all the residents of Foster's have gone missing. Bloo mentions literally everyone before realizing that Steve is not in his room, as he left with the rest of the house, except for Bloo, on a trip to the beach.