Yasu Takagi is a main character of the anime/manga, NANA. He is the drummer and manager of Black Stones.
Yasu is the Black Stones (BLAST)'s drummer, manager, and lawyer in training. He has been Ren Honjo's best friend since they were young. Yasu is naturally caring. He looks after the group and his other friends. Junko has commented that he seems to have reached enlightenment.
Pre BLAST period
Back in his high school days, Yasu dated ReiraSerizawa, Trapnest's singer and main star.
Orignial BLAST period
Black Stones was formed when Ren and Yasu invited Nobu Terashima to form a band with them. The name came from the cigarettes (Black Stone Cherry) that Yasu had picked up.
The period after Ren Left the Black Stones
BLAST reunited period
After seeing Yasu, Takumi Ichinose tells Reira that he must being chasing his real dream now. Reira comments that he never planned the band as more than a hobby and wondered what changed his mind. It's strongly implied that he reason is Nana Osaki and his feelings for her. Ren later reveals this to Reira just after a talk with Mari about lawyer training.
BLAST goes commercial
BLAST going commercial meant the end Yasu's of his career as a laywer. Yasu also would have had trouble continuing after he punched a paparazzi that was banging on Nana's door when the news of her relationship with Ren was announced.
In Gaia's "hotel resort", Yasu meets Miu Shinoda, an actress. She is envious of his relationship with Nana right away, wanting someone strong to be there for her. After a long string of events, Yasu and Miu begin dating.
Yasu is the one who smokes the cigarettes (actually cigars) called Black Stones, the name that was eventually used for their band. Yasu is first seen smoking them in the first volume of the NANA manga. When Nana Osaki asks him where he got them, he replies that he traded them with a tobacco salesman. Black Stone Cherry is an actual real label of small aromatic cigars by Swisher.
Yasu is rarely seen without sunglasses, however, in some rare occasions, he takes them off.