|Also known as||Nii-chan (by Momotaro and Isuzu Mikoshiba)|
|Height||187 cm (6'2")|
|Weight||77 kg (169 lb)|
Samezuka Academy (former)
|Relatives||Isuzu Mikoshiba (younger sister)|
Seijuro Mikoshiba (御子柴 清十郎 Mikoshiba Seijuurou) one of the supporting characters of the anime series Free!. He is a butterfly swimmer  and was the captain of Samezuka Academy's swimming team until he graduated, he gave away his position to Rin Matsuoka. He currently attends Hidaka University, where he is on the swim team alongside Haruka Nanase & Asahi Shiina.
Seijuro is a tall, muscular young man with short, slicked back, tousled, red hair and golden eyes. He has a darker complexion compared to others and is well-built from swimming and other exercises done at Samezuka Academy. He wears Samezuka Academy's black jacket most of the time. While swimming, he wears black swim briefs.
Seijuro is an enthusiastic person, being incredibly weak against cute girls - as seen as he has fallen in love with Gou Matsuoka on their first meeting, calling her “cute”. This attraction goes as far that his room is full of Gou's photos, as Haruka, Makoto Tachibana, Nagisa Hazuki, Rei Ryugazaki and Gou discovered then they went to visit Rin's room, but accidentally entered Seijuro's. Nagisa once stated that he had thought that Seijuro was only an enthusiastic person and that he hadn't been a good swimmer, however, this was proven wrong when Seijuro won 1st place in the swimming tournament.
Contrary to his happy-go-lucky personality when around things he likes and girls, Gou mainly, Seijuro is a strict, serious captain of a swimming team. Even though he's strict, he sometime tends to give comic reliefs in the show, such as when he told The Swimming Team that a fearsome new face will enter the club in April and started frantically laughing, making Rei wonder "what kind of villain is he supposed to be". He is said to defy logic, being able to swim 100 meters one hundred times, not loosing one single time.
Though he scolded Rin at the relay against the Iwatobi High School Swim Club at the regional tournament, in the end he is the kind of person who laughs it off and just forgives. To hold the swim team of the renowned Samezuka Academy together you have to have a big heart. However, he has no mercy for those who slack off. Though his eyes wander when he's in his flirting mindset, he is always firmly watching his club members’ movements, their bodily difficulties naturally as well, but he's also checking their mental conditions. When he assesses someone to be unfit, he doesn't shy away from pulling them out of the team.
Mikoshiba set a new record at the prefecture's tournament. Even as the champion school's captain, that ability is unfathomable. Even when that ability only vouches for his usual state of impetuous babbling.
- Main article: Seijuro Mikoshiba/Story
- Main article: Seijuro Mikoshiba/Relationships
The director of Free!, Hiroko Utsumi, said that since he had had to deliver some meaningful words to Rin during the endgame they had made him cool, but since he had been the captain they also gave him the sort of grandiosity and big-heartedness that comes with a "gahaha" laugh. However, she said, in the second episode, Kenjiro Tsuda's improvisation of the line "You're really cute" had really seemed to fit the character as well, so they kept it. The character designer, Futoshi Nishiya, added that Seijuro's design is the same as when he had first imagined him. While carrying a very captain-like majesty, he thought he became the most manly out of all the characters in the series. To match his appearance of the warm senpai, the staff made his hair red. There was a bit of orange mixed into that red as well, and so the staff immediately agreed on his hair color.
- It is shown that Seijuro has a strong similarity with Mikoto Suoh from K, as they both have red hair, golden eyes and the same voice actor, Kenjiro Tsuda.
- Seijuro reads Monthly Muscle Magazine, just like Gou does.
- Seijuro's room number is 201 and is across the hall from Rin and Aiichiro Nitori's.
- He has no official motif animal as of yet, but fans often link him with the swordfish.
- In the Free! drama CD "There's No Way My Little Sister Is Dating Some Guy", it is revealed that Seijuro has a little sister.
- His hobbies are karaoke, tennis and beach volleyball.