Ryuga Banjo (Kamen Rider Build)
Honestly, he is my favorite secondary kamen rider by far. All of his forms are imo, top tier (except for great Cross-Z Dragon I guess). He also has one hell of a backstory, which I won't spoil. Burning My Soul is also a banger.
GLaDOS (Portal)
Honestly why has no one used her in a chat group fic before. She is THE perfect person to be operating such a thing. Her dry, bitter and sarcastic sense of humor would work in most of the fics. I still can't believe I haven't seen her yet.
Literally anyone from team fortress 2
(Rant)
Listen, I get that people like an all-girls cast (except for the protagonist of course), but that is BORING in my opinion. Most of the time, the harem never gets fleshed out and it feels like reading the same chapter every time. And don't get me started on people who puts important characters on the backburner and only mention them every now and then. I absolutely despise those sort of fics. If you want us to care about the character, actually include them with the MC.
So, back to tf2. These characters don't just provide comic relief, but they have a really good character and background. They will add a lot of substance to your fic (if done right).
Sam & Max (Sam&Max) [why the fuck does the "and" symbol look like this???)
I'm not going to lie, these characters are probably REALLY hard to write. Sam himself has really complicated dialogue while Max is just an anarchist. If you didn't know, Sam&Max is a point-and-click game franchise. They are pretty fun and these 2 have been my favorite characters in poker night 2.
Off-topic
I miss RWBY fics. There is a good one right now, but man RWBY fics were really good.