Mike Schwartz is the catcher for the Harpooners. He's good at coaching and catching, but not much else.
When we first meet Mike, he's unnecessarily cruel to Henry, a shortstop he's never met on the opposing team. Henry can't hit the ball, so "Schwartz said—ever so softly, so that it would seem to come from inside [Henry's] own skull—'pussy'" (1.5). Mike soon witnesses how good Henry is at playing shortstop. When he sees that he can use Henry for his own personal gain, he recruits him to Westish and gets him on the baseball team. We're told, "Now that he'd seen that kind of talent up close, he couldn't let it walk away" (1.12) and "When he wanted something [he] took immediate steps to acquire it" (1.28). This is a protagonist we're supposed to like?
Henry starts college at Westish and, unsurprisingly, is ignored until Mike needs him. Then, to Mike's credit, he grooms Henry into a stellar baseball player. He teaches Henry to lift weights and to eat SuperBoost Nine Thousand to build muscle. They train together non-stop. And Mike starts a fight with the other shortstop to get that dude suspended so Henry can play. But, as we've seen up to this point, Mike is doing all this not because he likes Henry, but because he wants to win.
At some point, though, that changes. Mike yells at the umpire who marks Henry's failed throw an error, even though neither Mike nor the Harpooners have anything to gain by Henry's statistics. Maybe he does care about Henry and his record. "Get a new streak started" (23.25), he says, offering Henry encouraging words. When did Mike go from opportunist to friend?
Being a catcher is inherently self-destructive. It's an essential position—catching the ball and, metaphorically, catching all the players as they fall. But it's one that requires the player to squat constantly, putting huge stress on his knees. As a result, Mike is addicted to pain pills. Or, at least, it seems that way. He magically gets over his dependence with no trouble whatsoever about two-thirds through the book, so maybe he's making a bigger deal of the pain than it actually is.
This self-destructive tendency of Mike's spills over to other things, though. He's a wonderful coach, but he disparages his own skills, saying, "Those who cannot do, coach" (34.108). And when Henry starts to achieve success, Mike is jealous of it. Mike himself isn't successful, not getting into any law schools, but as Pella points out, "Why'd he only apply to six schools? The six best schools in the country? It makes no sense" (55.28). She believes he did this as yet another method of self-destruction.
In the end, even though Mike told himself "He had no art to call his own" (66.11)—as opposed to Henry and his art of fielding—Mike realizes that coaching is an art, and he embraces his calling, taking a position as Westish assistant coach. We hope that's a lot easier on the knees.