:) that thing you're talking about is probably mikumoi. a friend of mine, that's her oc. chance of her spawning is 1%. the reason why it's there is because i was too attached to that image when it was a placeholder for enemies, so I decided to keep her in a game as an easter egg.
however, there is another easter egg character. with 0.01% chance MIMI.EXE can spawn. it's op and kills you instantly, but it's unlikely to ever encounter her, I did though and that was amazing xD