Ren Mihashi (三橋 廉 Mihashi Ren) is the main character of the Big Windup! series. He is Nishiura High School's baseball team's current pitcher and ace and formerly Mihoshi Middle School's baseball team's pitcher and ace, whose nervousness and extreme lack of self-confidence is at odds with his skill, a result of his passion and dedication to pitching.
During middle school, Mihashi was bullied by his teammates on the baseball team after gaining a place on the team apparently through favoritism (due to his grandfather owning the school).
The other members of the team were resentful because of this and felt that their reserve pitcher, Kanō, was far more talented than Mihashi. However, Mihashi refused to give up the mound and pitched in every single one of their games, even when his teammates stopped trying, and the catcher wouldn't give him signs to throw anymore. Eventually, Mihashi decided to change schools for high school and succeeded in gaining a place in Nishiura. He felt guilty for ruining his junior high team's enjoyment of the game and promised himself that he won't play baseball in high school; however, he let it slip that he was a pitcher whilst visiting the baseball club and was thus dragged into it, given that they had no other pitcher.
Although Mihashi is convinced that he has no talent, he has been practicing hard at pitching since elementary school, and his new teammates, in particular the talented catcher, Abe, are astounded at his high accuracy. When pitching, he divides the strike zone into nine sections, which is said to be above the level of a professional pitcher, who would only divide it into four. Mihashi also has four breaking balls, including an unusual fastball, which often confuses opposing batters and a slider which drops a little. His pitches are said to be quite slow; although with training from Momoe, his speed increases later on in the series. He is also starting to learn how to pitch the "proper" fastball (with backspin) and throwing the ball from the windup position.
Mihashi is often seen anxious and has a very low self-confidence. He is shown to have always had this personality; however, it became much more pronounced in middle school during the time when he was bullied by his teammates on the baseball team. As such, he has struggles on saying his opinion which often leads to Abe's irritation.
Despite his low self-confidence, Mihashi is very stubborn, refusing to give up the mound even when so exhausted that he starts losing control of his pitches. He also feels upset at the idea of the team needing a reserve pitcher, going as far as having Tajima write the number "1" on his uniform's back in order to accept the idea half-heartedly. He loves pitching so much that his mother mentions he's always holding a ball, even in his sleep.
Mihashi's grades are very poor, and he says he had to work hard in order to get into Nishiura. Despite his clumsiness, he is also shown to have a talent in cooking and is confident enough to tell Abe what to do in the kitchen. He is quite gullible and loves to eat.
Mihashi's relationship with Abe as a battery is quite strained in the beginning, with Abe quick to lose his temper and Mihashi fearful of angering him. Mihashi greatly respects Abe and is eternally grateful to him for wanting to be his catcher. He notes that if it weren't for Abe's skill at reading the opposing team and calling for the right pitches, Mihashi could never make anyone strike out. Abe appreciates how hard Mihashi works on his control and his dedication to pitching for the entire game, though Mihashi still firmly believes it would be nothing without Abe, to the point that Abe promises not to get sick or injured for their three years of high school, in order to be able to catch all of Mihashi's pitches. Later on in the manga, they are seen communicating better and without the strain they had at the beginning, as Abe begins to understand Mihashi better, and Mihashi, in return, speaks up more and properly toward Abe.
He gets on well with the rest of his teammates, particularly Tajima, who always seems to understand him even when Mihashi can only stutter a few words, sometimes even by fidget. Tajima often acts like a big brother to Mihashi, protecting him and getting angry if he thinks Mihashi is being bullied. It is also shown that Mihashi would hang out with Tajima often enough that his grandfather as well as family would call Mihashi by his first name.
He and Hamada were good childhood friends. They lived in the same apartment building until Mihashi moved away. They would play baseball together, and Hamada would give Mihashi his old glove when Mihashi couldn't get one. They don't talk as much in the present, but their childhood friendship toward each other has yet to disappear.
He is also close to his cousin, Ruri, having lived with her family during middle school, though he dislikes it when she calls him 'Renren'. It is stated that back in his middle school days, he often told her not to come to his matches, thinking that he was going to lose and make himself embarrassed in front of Ruri. However, this changed as Ren entered high school, as he specially called her before their match with Tosei, asking if she could come.
He is still friends with Kanō in spite of being rivals for the position of pitcher in middle school. Although he became more formal with Kanō in school, likely out of guilt for not giving up the mound to him, Kanō has always thought that Mihashi was a better pitcher than him. They are now on good terms again, and Mihashi calls Kanō by his childhood nickname, 'Shuu-chan'. In the manga, after Mihoshi lost an important game, he messaged Mihashi, telling him about it and that he, too, will aim for Koshien next time, and as such, they became rivals anew.
- (About Abe) "It's still morning and I've already been praised."
- (To Abe, in his mind) "Thank you, Abe-kun!"
- (To Abe) "It's not like I'm not scared at all, I mean my heart is pounding so fast thinking I might get hit. But it's a totally different kind of heart-pounding than it was in junior high."
- Initially, Mihashi wears panda-like undershirts (used in middle school) but he later switches to pitch-black version due to the school's recommendation.
- He has a fear of dogs.
- He takes 15 minutes to go to school.
- Surprisingly enough, he likes people in nurse outfits.