Kobaltic

Demon form: (Found on Pinterest. He's the triangle demon in the left. Reverse Dipper is on the right. I'll draw Reverse(Underswap) Bella later)

Reverse Bill is actually a sad little Dorito (Sorry Bill but it never gets old XD) and isn't as evil as normal Bill Cipher from Gravity Falls.

Human form: (by bluedreamer0802 on DeviantART)