Like his fellow members of the Swimming Club, Sotsu Tanegashima and Mogana Kikaijima, Yakushima is obsessed with money, and is willing to go so far as to risk his own life to get a hold of it. Kikaijima claims this attitude originated after his family split up because they didn't have money. Like his fellow teammates, his dream is to fill an entire pool with money, and spend a whole day swimming in it. Nekomi Nabeshima labels him as the type who never reveals what he is thinking, nor what he wants to do. Though he considers money more important than his friends, he likes his friends more than money. Both he and Tanegashima think of Kikaijima like a daughter, and are prepared to make anyone who tries to lay a finger on her regret it.
Yakushima has dark purple hair (black in the anime) and a goatee. He wears a dark blue swimsuit. When not in the water, he wears the standard men's uniform of Hakoniwa Academy. At the Seiyasai, Yakushima wears a dark colored scarf with his uniform.
Because they didn't have money, Yakushima's family split up.
Student Council Executive Arc
Yakushima and the other members of the Swimming Club are first introduced discussing the upcoming club battle swim meet. Yakushima calls it an event made just for them, and encourages the others to enter. As a group, they decide that when they win, they will be one step closer to reaching their dream of filling a pool with money. Come Sunday, all three are seen looking pleased when Medaka Kurokami announces that any club that ranks higher than the Student Council will receive money from her own pocket. Yakushima agrees with Tanegashima when he claims that in the water, they are better than the beast Medaka, and Yakushima tells the group that anything other than first place is out of the question.
After the first event ends, the Swimming Club is seen on the sidelines. Medaka points out that they were actually the first group to score all twenty points. Other onlookers discuss the Swimming Club, one claiming he saw the guys swim to the pool's bottom, even with floaters on their arms. Medaka confronts the club, identifying the technique they used as compulsed expiry diving: if they had made a single mistake, they could have drowned. She asks them if they value their lives, a question Tanegashima answers on the rest of the club's behalf: they value money more than their lives.
Yakushima and Tanegashima take part in the second event, the three legged race.  When the race starts, the Swimming Club initially lags behind, due to Yakushima needing twenty-five meters to match Tanegashima's speed. With the preparations complete, Yakushima apologizes to Tanegashima for making him wait, who tells him it's not a problem. The two then begin swimming with their legs tied together, easily reaching the finish line first. After the third event, the Swimming Club confronts the Zenkichi Hitoyoshi and Kouki Akune. After Medaka claims that she will reform the Swimming Club, Tanegashima haughtily dares her to go ahead and try.
As the fourth event starts, the cavalry battle, Yakushima and Tanegashima are seen as the horse supporting Kikaijima, with Yakushima in the front. When Tanegashima asks him what they should do, Yakushima tells him that they have forty-eight points and the Student Council has thirty-three: if the Student Council gets their headband, they will be in first place. However, if the Swimming Club takes the Student Council's headband, they will have a fifteen pint lead on the Track Team, who are in second place. Even if the Track Team gets the headband of the Orchestra, who are in third place, the Swimming Club will still be fine. Yakushima deduces that the rules were not made to give the lower ranked teams a chance; they were made to force a dual between the Swimming Club and the Student Council. He agrees with Kikaijima when she states they have no reason to fight, and he suggests they get going. When Kikaijima declares that she will take that "stupid girl" (Medaka) down, Yakushima can only timidly agree with her. When Medaka is pushed off, Yakushima thinks to himself that it is a shame they weren't able to get the headband, but that with the Student Council out of the way, the other teams won't pose a threat to them. When Medaka knocks Kikaijima off balance and kisses her, Yakushima is shocked. When Medaka comes out of the water carrying Kikaijima, Yakushima takes her. When Kikaijima asks the two of them which is more important, her or money, he agrees with Tanegashima that money is, and reminds her not to be led astray by Medaka's influence. When Kikajima goes on to ask which they like more, her or money, he and Tanegashima share a look, before both tell her that they like her more. He is shocked with everyone else when Nabeshima and the Judo Club are revealed to have won the competition, and thinks that Nabeshima was surprisingly cool about being cheap.
Some point before accepting Medaka's offer to join the Student Council, Kikajima approaches Tanegashima and Yakushima to ask their advice. When Tanegashima tells her to join them, Yakushima agrees with him, telling Kikaijima that if she doesn't make friends her own age, the next years of high school will be hard for her.
Thirteen Party Arc
Yakushima addresses a sleeping Nabeshima as the Judo Club Captain, asking her if she isn't interested that Medaka is fighting on the senior floor. As the students watching Nabeshima punch Myouga Unzen believe she is fouling, Yakushima comments to Medaka that strikes are actually a part of Judo, and that it would only be a foul in an official match. When Myouga gets behind Nabeshima, Yakushima looks concerned. When Medaka moves to try and help Nabeshima, Yakushima stops her, claiming she has no right to help as she was the one who got Nabeshima into the situation in the first place. He tells Medaka not to worry however; pointing out that not a single senior student present is showing concern, and that Nabeshima didn't take the title of "Foul King" just because it sounded cool. As Medaka once again points out that Nabeshima is fouling, Yakushima repeats that fouling only matters in an official match. Yakushima asks Medaka if Myouga just called Nabeshima a cheater. When Medaka questions if he has deciphered Myouga's language, he replied in the negative, claiming that every one of Nabeshima's opponents have said that at some point. As Medaka leaves, Yakushima watches her go with Nabeshima.
Kumagawa Incident Arc
At the end of the semester, Yakushima is present for Medaka's speech. He is surprised by Misogi Kumagawa's sudden entrance, and declaration that he is Medaka's ex. He asks Nabeshima about the rules Kumagawa is quoting. He is surprised to hear the academy used to be just a cram school. On August twenty-second, Yakushima is among Medaka's former enemies who show up to cheer for her during the President's Battle.
Kurokami Medaka's Successor Arc
Unknown Shiranui Arc
The Swimming Club are approached by Medaka asking about Hansode Shiranui; none of them remember her. At the end of the academic year, Yakushima is among the graduating students present at the Hakoniwa Academy graduation ceremony.
Bouquet Toss To The Future Arc
Yakushima is among the students who confront Medaka at the beginning of the 100 Flowers Run, holding flower number eighty-eight. He and the others are all blown away by Medaka's Kurokami Final. All present wonder if Medaka thinks of their time together as good memories. His message to Medaka is to swim through life.
Ten years later, Yakushima and Tanegashima continue to swim, now doing so professionally.
Swimming Prowess: Yakushima is a Special and one of the school's many scholarship students. Yakushima is Captain of the Swimming Club; his talent is widely recognized, even outside of the club, and he is known as a very well rounded swimmer who can use all kinds of strokes and techniques.