(Copied and pasted from the Reverse!Falls Wikia)
Reverse Bill, also known as William Cipher, is the reversed version of Bill Cipher. Will is powerful, considering his species, and specializes in the realm of the mind. However, since he is not at his full strength, he is somewhat of a pushover.

The twins see William to be their personal servant or pawn. William was summoned by Mabel and Dipper and was soon trapped working as an enslaved demon.

Personality:

Unlike his counterpart, Will is clueless and naive, something the twins take great pleasure in when toying with him. He is scared easily and is somewhat of a coward. However, this is usually applies only when he is with the twins. His worst nightmare is likely the Gleeful twins, as they tease and torture him with their telekinesis, stabbing him, chaining him to walls and pulling on his body.When he has finished whatever he was meant to do, the twins put him in a powerful cage that only they can control.Will was summoned at first to rid the twins of Gideon Pines, and after the job was done, they captured him and proceeded to use him as a toy for their entertainment.

However, Will is the only one to know the twins' secrets, and if he were to escape and tell someone, the twins would find him and likely torture him worse than usual, before keeping him in a cage for the rest of his life.

Will gets upset very easily - it doesn't take much to make him cry.

Extra:

William Cipher is also sometimes referred to as Bill's twin brother rather than his counterpart.

William also has many nicknames, sometimes from the Gleeful Twins, Pacifica, or fans themselves; some examples would be: Blueberry, Bluebird, Cinnamon Roll/Bun, Demon, and Cool Ranch. Often, the Gleefuls (being manipulative and abusive owners) refuse to call Will by his real name, and simply refer to him as "slave", "useless demon", and other insults.