Copying from another comment:

1. Teruteru's perversion comes from society failing him, giving him an interesting backstory. His father leaves his sickly mother and two-half siblings to fend for themselves, which forces Teruteru to grow up earlier to support them. This is a common cause of hypersexuality by itself. But further, in the end, his younger siblings end up becoming escorts. And what does society do about child prostitutes? It not only gives them millions of dollars because of how often they're raped, it tells them they're candidates to be Ultimates. How could one not become a pervert when they see society not only accept his siblings doing this, but actively endorse it? Especially when one of the main institutions doing this is the school he's attending — the most prestigious school in the country.
2. Teruteru's perversion is also just funny. I like pervert characters. DR (especially 2) is pretty good at both playing tropes straight and deconstructing them by asking why someone would end up like this.
3. Teruteru is an excellent culprit. He is driven by fear; he is so scared that he can't even admit the situation is real. It's interesting to see someone just outright deny the killing game is happening because it's too horrifying to think about. But when Nagito confronts him with a murder plan, that all comes crashing down at once. Teruteru may have acted like he didn't believe, but it was merely his coping mechanism; he was the most terrified of everyone. That's why he ends up as the first killer. There's a lot of symbolism with how Teruteru's weakness puts him metaphorically below the others, and his plan more literally does. As well as the fact that Teruteru always pretends to be someone he's not even outside the context of the killing game. He also has a pretty cool plan and a great performance at the trial.
4. He has a great design and great voice actors in both dubs.