The beauty of Abe and Mihashi as a battery (and pair of unlikely friends) is a common formula behind most pairs that I enjoy: opposites attract. The best kind of relationships are the ones where both come out better people because of their ability to learn from one another's differences—Abe and Mihashi are exactly that. Abe is the loud, hot-headed, confident catcher, and Mihashi is the shy and self-deprecating pitcher. Together, the two of them learn to work together to form Nishiura's unstoppable battery, but more importantly, through their differences they help each other overcome some of the most difficult insecurities carried deep within themselves. (They also hold hands while blushing, cry in each other's arms, stare longingly at one another across the field, promise to never get injured so they can always play together... all those adorable things boys in sports anime do to make us chicks go crazy with gif sets and conspiracy theories on Tumblr and shit.)
"You're a good pitcher, even though you irritate me and piss me off. And I like you, not just as a pitcher, but as a person."
Abe enters Nishiura with a bit of a stigma against pitchers: he previously played on a team with the cocky and incredibly aggressive Haruna, who threw balls so hard, Abe would be covered in bruises after a game. Though he tolerated the pitches in order to improve as a catcher, he grew to hate Haruna, who would selfishly walk out of a critical point in a match if he had reached his limit of 80 pitches per game. Abe grew frustrated playing with a teamate who had no interest in the team, but simply used games as his own selfish form of practice.
On the flip side, the timid yet stubborn Mihashi has no idea what it's like to have a supportive team or catcher to play with. As the grandson of the owner of his previous school, his teammates believed that he gained a place on the team purely out of favoritism. Despite this, Mihashi stubbornly continued to pitch every game due to his intense love of pitching. His teammates grew to hate him and his catcher would eventually even stop throwing him signs out of frustration. Mihashi lived with this guilt, but was never able to better himself as a player due to the fact that his teammates refused to cooperate with him.
Both Abe and Mihashi have a "history" with their past battery, but I think the two of them both know that Nishiura grants them a fresh start. Though initially Abe looks to take advantage of Mihashi's doormat personality, over time, he learns more about Mihashi's motivations and all the hours he puts into practicing. Rather than simply manipulate Mihashi and use him the way that Haruna did to him, Abe opens up to the idea of working together as a team. Given that both of them deal with these insecurities, the relationship that unfolds between them during their time on Nishiura truly helps them overcome those difficulties from the past. They learn to be part of an environment which breeds much healthier relationships and can truly enjoy the game they love so dearly in its purest form, without all the politics and drama. They work together to achieve their goals, not against one another: Abe even promises Mihashi that he will make him a pitcher worthy of being called the ace—you know he da bae when he's not only chasing his dreams, but yours too!
More importantly than growing as players, they grow as individuals. And more importantly than becoming reliable teammates to one another, they become friends. The most beautiful thing that these two do for one another is help heal those wounds of the past: Abe helps Mihashi to find confidence within himself, and in return, Mihashi gives Abe the trust that he thought he didn't deserve. The way that these two consistently think about one another on and off the field is incredibly sweet (probably even borderline romantic, much to the pleasure of BL fans everywhere), but it truly speaks to the relationships that Nishiura is allowed to breed because of how close they all are as teammates. Watch Oofuri for the baseball, but stay for the adorable friendships. Nishiura is #squadgoals. (Seriously. I rewatch it probably bi-annually. I don't know how there hasn't been a shrine written yet. I'm actually disgusting.)