So Sorry is a secret boss in Underfell.
He is first shown at Hotland in the art club room.
The Underfell version of So Sorry looks the similar to the Undertale version but wearing a black vest, he might also have orange eyes and red pupils, similar to Toriel and Asgore. But somehow more menacing.
Unlike the original So Sorry, the Underfell Sorry appears to be selfish, rude, lazy, and stupid as said by Flowey. He appears in the art club room in Hotland. He appears to be aggressive too. He hates mercy and he likes to be alone and hates humans.
He runs into Frisk on purpose and tries to kill them.
In Undertale, the computer clock needs to be set to October 10th to fight So Sorry. However, in Underfell he can be fought any time.
Another difference is that he can be fought while on the genocide route.
The fight begins with So Sorry running into Frisk and trying to kill them. His first attack is using his tail. He also uses paper balls and uses Doodlebog to kill Frisk, but instead heals them, which makes him very annoyed. The fight ends with So Sorry running away from Frisk.
So Sorry doesn't appear in Underfell due to some controversies based around the character due to the creator of the character having weight gain fetishes.
So Sorry is killed by Frisk, which causes him to melt to his death.
True Pacifist Route
Frisk shows mercy to So Sorry. Since So Sorry hates mercy, he runs away from Frisk, ending the fight.
Frisk - He hates frisk cause they use mercy.
Flowey - Flowey is a jerk to him for some reason, even though flowey is scared of most of the monsters. Flowey somehow hates So Sorry for being dimwitted. Flowey said he did meet him before, meaning that he could have been hating him before frisk fell down.
Alphys - Alphys is So Sorry's possible sister (noncanon), since they could be the same creatures. But since Alphys is a lizard and Monster kid is a dinosaur And Sorry is a dragon. It's pretty unknown.
Monster Kid - Possible brother (also uncanon) since they could be the same creature. But also not known since he is a dinosaur and Sorry and Alphys are a lizard and a dragon.