Like Sans, he’s vengeful and will call you a “Dirty Brother Killer” if you choose to kill Messy; in the No Mercy fight, he’ll be no joke. He’s NOT going to show ANY mercy and if you spare him when he offers his “Mercy”, he’ll dunk you.
He loves to make puns and sells Hot Dogs in Hotland.
He hangs out at Tinsle's and his mother's Resort. He knows his family like the back of his paw.
He is able to see every timeline, has met with Sans and was warned about a "Dirty Brother Killer", who will one day stab William & his friends in their backs on a "No Mercy" Route; though William discarded this, he still has faith in Sans about his part as Judge.
While Scar is chasing you, when you pass William, Scar will be JUST as mad at William as Messy is for being lazy. William will judge you for your actions in each route before you go to fight Master Tactician, DoomFlame.
He also has an optional Boss Fight in his father's Lab, hosted by Mizukiton EX after her battle at the CORE where his battle theme, Song That Sprouts if You Fight William, plays during the True Pacifist run.
- The reason he wears diapers is failure of his potty training back when he was on the surface and was still with his Wife, Kaleidoscope Kageyuki Freeze.
- His Swap Self likes to eat Salad.
- The selected music is made by the Ninety9Lives music label.
- William's Megalovania Theme "LEAFEONOVANIA" is the Elektronomia - Sparks tune.