This is a version of Bill from Gravity Falls from an alternate universe known as Reverse! Falls, commonly referred to as Will Cipher. The version of him that I play may differ from other fan-depictions, but he was originally something of a hero in his home dimension, responsible for sealing the party demons in that prison dimension, and typically able to come and go as he pleased-in tales and legends of humanity of old, being thought of as something akin to a 'blue angel', benevolent and kind.
Until Stanford Gleeful summoned him for his own ends-but finding that through some botched summoning or even failsafe, Will's summoned and bound to him but without much of his limitless power-no doubt to prevent it from falling into the wrong hands. Will's resiliant at first, but witnessing and experiencing Ford and the rest of the Gleeful's cruelty firsthand eventually erodes his previous fortitude, eventually reducing him to the frightened, tearful mess he's known as now. At first he feels like all hope is lost for him, moreso when Ford discovers the prison dimension where the other demons are, wanting to bring about Weirdmagedon himself.
However while Wll is floating through the mindscape, one of his few escapes at least in non-physical form, Will meets Ford Pines and learns of the rest of the Pines family in an alternate dimension, which sparks hope anew in him.