Smiler starts out as a less monstrous Trollge, relating more to what a normal stick figure would look like. However when pushed it seems he becomes more akin to a normal Trollge. Smiler constantly becomes glitched in numerous ways. In his final phase, he has the body of a normal Trollge but the head of his original self.

More facts here: https://fridaynightfunking.fandom.com/wiki/The_Trollge_Files