Today is April Fools' Day, so I draw the indie game character as the character that looks similar but also has a reason.
Reason of Costume

Sans (Undertale) as Helvetica (Helvetica)
Reason: Sans and Papyrus get references from Comic Helvetica

Baldi (Baldi's Basics) as Eggman/Dr. Robotnik (Sonic)
Reason: The game gets a reference from Sonic's Schoolhouse

Tattletail (Tattletail) as Furby (Furby)
Reason: Tattletail design is a parody of Furby

Spooky (Spooky's Jumpscare Mansion) as Fionna (Adventure Time)
Reason: Art Style similar to Adventure Time

Bendy (Bendy Series) as Grimm (Hollow Knight)
Reason: Both of them just look similar... Yeah, that's it.