Weib's Associates
Ah...everyone loves Weiß....Well, everyone loves Omi...and I guess we all just put up with the rest of them, eh? ^_~ Anyway, they're the heroic good guys of the show, so naturally they accumulate some friends and allies, most of which are young attractive girls, but who's counting?
Kritiker
Kritiker(Which means 'Critic' in German...I think)), the protectors of all that is pure and true ^_^...look how dramatic I can sound! Yay for me! Anyway, they are the larger organization that Weiß works for. Everythings pretty much a secret about them, but it's safe to assume that doing good is their main objective. Weiß is in contact with a few members of Kritiker. If you have seen the second season/half of the show, you shouldn't read about Botan and Birman, unless you like spoilers, since they don't show up until then.
Persia
Yes, the mysterious man on the video. He sits concealed in darkness and delivers Weiß's new assignments...very 'Charlies Angels'-esque. Anyway, after a while you eventually see his face, he's just a normal guy with a beard (I was hoping he had have some disfiguring scars or something that would make everyone go *GASP* but nnnnoooo...). Aya has a few talks with him, not exactly pleasant ones either. It seems like Omi has a relationship with him, and probably spends more time with him. It's pretty resonable considering Omi has no parents to raise him and Persia is his employeer. Of course there's more to Persia than meets the eye, his story is a twisted one of lost love (aw), betrayal, and death. This man also really, really wants to take Takatori down. On a lighter note, on another site I took a Weiss love match test and I matched up with...Persia!!! I almost had a heart attack when I saw that. 
Manx
Manx is cool, I love Manx!! I never appreciated her until Birman showed up. Up until then I was too preoccupied hating her hair and trying to figure out why she wore socks with high heels. Anyway, she's Perisa's assistant, she delivers his videos to Weiß and briefs them on their missons. She's the go-between for Weiß and Esset. I think she also knows everyone's secrets. Members of Weiss often confide in her and she knows all their pasts, including Persia. She also has to put up with Yohji and his constant attempts at picking her up. So after awhile, her hair isn't so annoying, infact, she looks really cool with her hair up and in a kimono as you see in one episode.
Botan
Miyako-chan's a Botan fan. He's only in a few episodes, but his presence is important. He works for Kritiker and is resposible for bringing Weiss back together. More importantly, he convinces the ever-stubburn Aya-kun to join again(Convince Aya-kun?!?! That's how you know you're good). Talk about powers of persuasion. Anyway, what's the point of having another male character on the show if he doesn't have an angst-ridden and tramatic past? Well, don't you worry, he's got a past as well, let's say that his daughter inspired him to help others. Oh yeah, he's got a thing with paper air planes because his daughter used to like them.
Birman
Ehhh...Birman is Botan's assistant, and she sort of replaces Manx. Not much known about her character. It seems like she and Manx are buddies though, so don't expect any vicious catfights over who's the best assistant. She also is willing to put herself in danger for a mission and has had a few unfortunate run-ins with Schwarz's resident pyscho Farfarello. I think Birman would make a good companion for Aya-kun, cuz they're both all emotion-less. Actually, after Birman showed up I started to appreciate Manx ^o^...Sorry Birman.
Momoe
This is the little old lady that also owns the flower shop the boys work in...either that or she just hangs out there A LOT. You don't really see a lot of interaction between Weiss and her though...She just sits there petting her cat and smiling. They worry about her though, and when they expect danger they make sure to keep her out of it.
Sakura
A pretty young track star whose kidney was stolen by a mad doctor working for Takatori Masafumi. Aya-kun protected her when the same doctor came after her heart. Anyway, she develops a little crush on Aya-kun, and he's pretty interested in her, mainly because she look identical to his sister Aya-chan!!! Ick and so the incest begins...She shows up in other episodes looking for Aya-kun at the shop when he isn't there and I didn't even realize at that the time that it was her. I just thought it was some girl. Anyway, she starts having a bigger part later in the second season.
Ouka
Didn't you just hate her when you first saw her? Ouka's the perfectly spoiled rich girl who is used to getting what she wants. And now she wants Omi-kun. And maybe after awhile he wants her back ^_^. You'll have to watch and see. She gets less annoying, but she's still ugly ^o^. Anyway, she lives with her mother who owns a restaurant, and her father didn't take responsibility for her until recently. But now they have a very loving relationship and he works as a politician. Wait until you see him meet Omi-kun...you won't forget it!