So Sorry is a mini boss in Undertale. TBH, he sucks. His fight is long and annoying, and you don't earn anything worthwhile from killing or sparing him.
Long story short, people could pay Toby a bunch of money to include their OC in Undertale. I'm fairly certain that Muffet was one of them. So Sorry is one as well. The person that made So Sorry also made him for a bunch of innapropiate stuff as well, I'm pretty sure, which I'm fairly certain is why Toby didn't make him one of the mini-bosses you have to face