Who's the catboy?
I've watched the Negima TV series (all but the last episode anyway... as far as I know) and I've never seen him... Is he only in the Manga or something?

Kotaro. Dog-boy. Has quite a role in the manga. First appears in Kyoto arc.

Wait, I thought he was in the TV series, if not I'm sure he'll be in the season 2. And yes, he is based on Inu Yasha, Akamatsu (Ai Ga Tomaranai, Love Hina and Maho Sensei Nagima!) has admitted that he is a fan of Takahashi's work (Urusei Yatsura, Maison Ikkoku, Ranma 1/2 and Inu Yasha) for years now, even Motoko's (Love Hina) attacks and those of Setsuna (Maho Sensei Nagima!) are based on Inu Yasha's ariel attacks.

Yeah, ninja girl Kaede just fights a bunch of demons when they do the Kyoto trip intead of fighting Kotaro and they never do the Nodoka's picture diary either which also involves Kotaro.

Nodoka's in High School, they don't do picture diaries??? And it involves dog-boy, WTH :D So in the manga that's Nodoka's guy?

Negima...NEGIMA!!! I'm sorry...the picture diary is an important plot spoiler, so no information for you and no Kotaro is no Nodoka's guy, she's still chasing Negi :p

They're in JUNIOR high. As in middle-school.

Some spoilers about artifacts...

You misunderstood about the picture diary, it's not an actual picture diary, it's Nodoka's artifact and it's very powerful. It just takes the form of a picture diary. :) Yue's artifact is the same way, it appears to be an introductory textbook to magic but it turns out to be very powerful as well.