Mamiya Seiji is first shown as an 18 year old college student at Reinan University, who is also a first year in karate, along with Minoru and Hayama. Mamiya was supposed to join the Kendo Club, since it's a family tradition. But with the feelings of not wanting to stay in his brother's shadow and amazed at Mutou's strength from beating his kendo bullies, he decided to join the Second Karate Club instead. Their father has a very lucrative construction company, which makes the family rich.
He has short black hair. He is short and has a small build. After the art evolution, his hair transforms into a bowl cut with medium length sideburns. The front of his hair is a little wavy. Thin full length eyebrows.
Mamiya Seiji is a kind boy who first helped Minoru when he was just new to the karate club. Mamiya cares deeply about his teammates. He was so worried that his older brother would beat Minoru up that he almost considered quitting the 2nd karate club and go back to to the kendo club. Mamiya is a stubborn boy at some times.
Loves his deceased mom and his family, even though he and his brother's relationship is strained from time to time.
Since he was little, he has always had a thing for blondes. This is due to the fact that his second mom was blond and she was always nice to him. Seiji can be seen having a crush on Eileen, Michelle Izumi, and Alyosha (this ends when he finds out Alyosha's secret). After the incident with Seiji and Alyosha, Mamiya starts to become more fond of brunettes.
He can be self-centered at times. He needs to accept things through theoretical explanation.
He is the type that is too attached to the idea of winning and won't even consider losing.
He seems to be a little jealous of Kohinata's strength and beauty.
When he was in kendo, in high school, he went to the semi finals at Gyokuryuuki. One of his weaknesses was that his kumite was just a monotonous repetition based upon pushing forward and trading punches. Which isn't a strategy for someone who isn't blessed with a strong physique. He used to move in head first without moving his legs. He has short legs, so he is bad at low kicks. In his second and third year, Mamiya improved greatly. He is fast and said to be the fastest in the club (most likely Minoru is and he's second).
On Christmas, a month before the Ragnarok tournament, he asked his brother for training. The elder Mamiya then teaches his younger brother a family secret kendo technique (Mamiya-ryu Kodachi Secret Sword Night Mist). His brother explains that Night Mist's greatness is all in abandonment of rhythm. Basically when the enemy's sword reaches 7/10 you thrust in the remaining 3/10. Waiting until 8 or 9/10 will get you cut down, and 5 or 6/10 the enemy can change the sword's trajectory and still cut you down. When the sword reaches 7/10 and enemy is certain that he's cutting you down, you thrust (in his throat) into that opening in his mind. From the angle it looks practically invisible, like a mist in the night. Moreover, if you want to perform this technique that's designed for a close maii kodachi, with your bare hands, you have to cling tightly to the opponent or the technique won't work. After two months, Seiji finally masters the technique. Seiji's bare-handed version of this technique is an uppercut. He has also built up a strong resistance to body blows from getting hit by a shinai (wooden sword), while Kiichi was training him.
After the Ragnarok tournament, he has further developed his Secret Night Mist technique and can perform it on three opponents at a moments notice (Secret Fist Triple Night Mist).
Seiji is also shown to have very great analytical skills and can give good advice during a match. He has given advice to Minoru on multiple occasions and has become his second a few time as well.
His older brother who's relationship is a little strained. Kichii forced Seiji to go back to the Kendo Club because it's in the family tradition. After a fight between him and Minoru, he let Seiji decide what he wants and to go back to the Second Karate Club. They never spend Christmas together. But he helped Seiji train for the tournament, K.O.S. Ragnarok, and taught him a family secret technique: Mamiya-Ryu Kodachi Secret Sword Night Mist. And secretly encouraged him at the tournament. He also secretly wants him to become the best in Japan in whatever he's doing.
They are practically brothers. However, Seiji harbors a secret rivalry with Minoru (even though Minoru doesn't realize) because of Minoru's strength and gains the most attention in the club (with the exception of Mutou/Ibuki) and more attention than Seiji and Hayama. He also tends to get very jealous of Minoru's success with the ladies- particularly when one of the ladies in question is a blonde.
Seiji wasn't romantically interested in Akiko, since she had feelings for Minoru, and Akiko didn't initially like him. He also thought she was weird. The two first met at a preliminary tournament after Mamiya's loss. Mamiya was alone and crying and Akiko was consoling him. They've been on friendly terms ever since. After the Alyosha incident, these two are seen in the midst of starting a relationship.
Like Minoru, Mamiya thinks of Hayama like a brother. They have a more apparent rivalry and is shown on multiple occasions.
Mamiya pratically worships the ground Mutou walks on, as he wishes to be like Mutou someday. He was first unintentionally inspired to join the Second Karate Club when Mamiya was getting beat up by Kendo club seniors until Mutou saved him and decimated the bullies. Seiji was astounded by Mutou's strength and has been a fan of his ever since.
- After 30 Volumes or so, Yasushi Baba begins to evolve his way of drawing muscles. It becomes more defined, shaded and solid; it coincides with the MMA Genre Shift as the characters now are mostly shirtless, and no less important, he begins to draw the women more curvy as well. Characters in particular got a beauty upgrade, Hayama and Seiji are glaring examples, their designs are completely renewed; it might be for matching looks with their beautiful girls, as they got Rika and Akiko as their Love Interests.
- He lives alone in a apartment filled with plasma tvs, amazing sound systems and stocked fridge.
- Hayama refers to him as a "rich boy".