Kobayashi Yuu is one of those seiyuu who doesn't really have a huge role that she's known for, but has played several recognizable supporting characters. Her most notable main roles are probably in recent shows like Mariya in Maria+Holic and Rio in So Ra No Wo To. Her lower voice range means that she's likely to voice characters who fit the strong female character archetype, which her more interesting supporting roles seem to indicate: Baccano's Nice Holystone and Negima's Sakurakazi Setsuna.
She has a voice that's on the deeper end of the spectrum, which lets her voice boys as well. Most recently, she was the main guy in Ookamikakushi, the pretty useless Kuzumi Hiroshi. Her supporting roles as guys is more interesting, both being of the When They Cry variety. Her two roles here are as the missing elder brother who's often mentioned, Hojo Satoashi in Higurashi, and the emotionless young servant who has the One-Winged Eagle, Kanon in Umineko.
I don't think she's an actual singer, but she did sing the OP and ED themes for Maria+Holic. I noticed that it seemed to take her out of her voice range that I was used to hearing. I don't think I would have guessed she was singing Hanaji if I didn't look it up.
She's also well known for her terrible, terrible drawings.
Major Roles
Hiroshi Kuzumi in Ookamikakushi
Kaere/Kaede Kimura in Sayonara Zetsubou Sensei
Kanon in Umineko no Naku Koro ni
Mariya Shidou in Maria+Holic
Rio Kazumiya in So Ra No Wo To
Satoshi Hojo in Higurashi no Naku Koro ni
Setsuna Sakurazaki in Negima!