His existence is only hinted at during the game until the pacifist ending finale, where he is revealed to have been with you all along as Flowey the flower. He is never seen until that point, and the very mention of his name, much less his appearance, is a spoiler for the game's pacifist ending.

Note: Due to TWYS guidelines, the character's forms are to be tagged separately (particularly his god form) unless they happen to be in the same post.