Tsurumisaki, along with the other members of the Plus Six, are unknown to Maguro Kurokami, indicating that they did not become a part of the Flask Plan until after he left.  He is a junior of Class 13 and a former member of the Flask Plan's Plus Six. 

Kurokami Medaka, a first year, is elected as Student Council President, and the first thing she does is establish a suggestion box, later dubbed by students the "Medaka Box". Medaka encourages students to submit any problem to the box without hesitation and promises to take on any issue from anyone. 
 
Since Medaka won the Student Council elections with a staggering 98% of the vote, she ends up as the Council's only member. She asks her childhood friend, Hitoyoshi Zenkichi, for help and he grudgingly agrees, becoming the Student Council's lowest ranking member. Soon Medaka recruits two more people to the Student Council, the former judo club member Akune Kouki as secretary and the current swim club member Kikaijima Mogana, on loan for 300 yen a day, as treasurer. 
 
Now the Student Council members spend their days solving the problems submitted through the Medaka Box, whether they be hooligans in clubs where they don't belong, girls who need help writing love letters or people who've lost their pets, gradually earning the respect and admiration of the entire school. 
 
Included chapters: 
Volume 15 contains the gaiden "Good Loser Kumagawa" (グッドルーザー球磨川); Jump NEXT (2011/12/26) 
Volume 22 contains the gaiden "Good Loser Kumagawa Final" (完結編).

What started as a simple assistance program has escalated insanely out of control, and now Medaka Kurokami finds herself exposed as something far more than human. But that's only the beginning of a new series of shocking revelations that lead towards a dark secret plan that not even the unbelievable power of her War God Mode may be able to withstand. Still, Medaka's proving to be good at thinking outside of the box, and if she needs the full backing of every hidden resource of the Student Council to meet the challenge, then that's what she's going to get. Even the Abnormal ones! 
(Source: Sentai Filmworks) 
 
Note: Episode 12, titled "Seitokai wo Shikei Shikkou Suru", is part 4 of the "Good Loser Kumagawa" series.

Yamami Tsurumisaki