Though the son of a family engaged in manufacturing sports equipment, Tobimaro has no athletic skills and extremely poor reflexes. His only special skill is swallowing and spitting up baseballs, sometimes spitting several out like a pachinko game. He also seems to lack some common sense as he is always misunderstanding things, such as when he declared the winner of his and Mendou's duel must "marry the other".
He is usually dressed in rags and has shaggy unkempt hair which covers one of his eyes. His eyes have stars in them, a parody of the "starry-eyed" heroes from 1960s and 70s sports manga such as Kyoujin no Hoshi. This appears to be a genetic trait of the Mizunokouji's, as even his great-grandfather and sister Asuka possess them.
He is the object of affections of his own sister, Asuka, but insists on seeing her only as his sister. Even though he is far weaker than the super-strong Asuka, he is very protective of her and even willing to use his own body to block "attacks" from Ataru and others. Unfortunately, everything he says and does only encourages Asuka's incestuous desires, leading their mother to refer to him as "Good-for-Nothing" and a "Worthless Older Brother". Asuka will often sneak into his futon at night and walk in on him as he is bathing. After seeing her naked body once, he lost 2/3 of his blood through a nosebleed eruption, and his bones are often broken by her tight hugs. Doctors always remark on how he is still alive after such trauma, implying he may have similar resiliency to Ataru's.
Although they call each other rivals, he and Mendou have been friends since childhood and still refer to each other by their childhood nicknames Ton-chan (トンちゃん, Ton-chan?) and Shuu-chan (終ちゃん, Shuu-chan?). They have been having an annual baseball duel for years, but have yet to reach a conclusion. Because of his poor reflexes and baseball skills, Tobimaro can never throw the ball correctly and, as a result, Mendou can never hit it.
Despite having access to great wealth, he is completely different from Mendou. He wears a ragged budougi and carries around his baseball bat in a sack over his shoulder. He spends the whole year training in the mountains, but his training is rarely related to baseball. He also developed a habit of eating baseballs while living in the mountains.
His reflexes are possibly the worst ever. He is capable of climbing tall trees, but lacks the ability to climb down on his own, requiring a ladder he carries around with him, which often falls or is knocked down while he is still in the tree. However, he is really fast at running away and has, on occasion, been able to catch Mendou's blade between his hands (much like Ataru always does).
Tobimaro dislikes women, and this seems to be due to his violent history since childhood with Ryouko. He has suffered a fair amount from her and as such avoids her at all costs, but unfortunately runs into her a lot anyway. Ryouko claims to love Tobimaro, but based on her personality it is never made clear whether she is honest or just toying with him. Mendou object to their mere association and never seems to understand Tobimaro's true feelings towards Ryouko.
Despite his hatred of women, he associates with Mendou and Ataru, who are connected to many women, and his own personal subordinates are all women. He is also still a young man, and will nosebleed whenever he sees a naked woman and was depicted with hearts for eyes when he sees Lum batting.