This is so tricky. I like so many of the characters because they feel unique and real compared to nearly everything else in the demographic. The character designs don't hurt either.

Rin - He feels incredibly genuine and "refreshing". It feels good to see him make friends despite his heritage, and learning to control his aggressive tendencies (though I assume that happened offscreen).
Yukio - Oh god where do I start? XD I, also, will enjoy his eventual breakdown... and want to see him come back from it better than ever XD. I think I've said it in another thread, but I'm waiting for him to inherit Satan's flames too. His feelings toward his brother and the flames are contradictory (on one hand, he is jealous of his brother's powers so at a very deep level he must want them too. On the other hand, he is so afraid of them that he's starting to crack at the very possibility of that happening.) There are other things to consider I guess. I can't really read characters well so this is fun to try figuring out.
Bon - I just like this guy. He's hyper competent, he's interesting, he has awesome hair.... and his personality is very "strong" if you get my meaning.
Shiemi - I love the way Kazue Kato is playing with the moemoe character archetype right here. She tries so hard and she's succeeding at becoming a great tamer.
Konekomaru - The Kyoto .... Uh .....
duo..... Seem to be the brains of the team, and he's been coming into his own recently. Maybe he and Shiemi should multiclass to doctor.
Shima - Not that I necessarily hate Shima. I look forward to seeing which direction his character goes in
Izumo - She seemed like a bitch. Until I reread the series from the beginning. Really, she has so many trust issues and she hides it so well. Knowing everything up to ch48, I can't blame her for the way she's behaved (and understand why she has nice moments littered throughout her interactions with the other classmates.) and after that chapter I just feel so sorry for her. But she should ....probably join Yukio at those psychiatrist sessions someone was talking about. I bet she's even more unstable than he is right now.
Mephisto - I honestly can't decide if I love or hate the guy. The only thing disappointing is how frequently he is demoted to Mr. Exposition. But holy shit the FABULOUS ~
Angel - And the runner up for the fabulous award iss~~
(I like fabulous people. Can't you tell?)
Hell, why not Amaimon too? He hasn't really done anything except fight so I want to see more of him.