Okay, by side characters I assume you mean occasionally recurring ones, and not the main supporting characters. Let's see... off the top of my head...
Tetsutetsu - I like his friendhsip with Kirishima and straightforward sense of honor.
Hawks - I think the character is cool enough, but the voice acting in the Eng Dub for this character is brilliant, I think. It really gives of that cocky charm and dry sense of humor well.
Twice- Pretty dark backstory, but the whole split identity thing makes him a perfect vehicle for comedy.
Gentle Criminal/La Brava - I mean, these two are just such "characters". Brilliantly theatrical and whimsical.
Rappa - The guy may be a villain, but he is an honest one. Loved his fight with Fat Gum and Kirishima. The poor dude just wants a simple, traditional "fight to the death".
Miss Joke - More of her annoying the hell out of Aizawa, please.