None of the other Harpooners have much of a personality, if they even get any lines or mentions beyond one or two plays in a game. Adam Starblind seems to be a big-talking womanizer. He's also the pitcher (although it is unknown if he has a big butt). We also occasionally hang out with Rick O'Shea and Izzy Avila, who we're told "worshipped Henry" (5.94). We also spend time with Mike's roommate, Arsch, who is seen in only about five paragraphs yet manages to scratch his testicles twice (charming).
Then there are people like Quisp, who must be named after a box of cold cereal, and Sooty Kim, whom we guess is a former chimney sweep. There's even someone named Craig Suitcase, who grumbles and spits sunflower seeds.
Two seniors are on the team when Henry is a freshperson, and they seem to exist to foreshadow Henry's decline. Jim Toover is only on the team briefly. We're simply told, "He chokes" (5.11). Then there's Lev Tennant, Henry's early rival, who also foreshadows Henry's fall from grace. Someone says, "Maybe if somebody told Lev he didn't have anything to worry about, he'd relax and play better" (5.55). Lev has Henry to worry about, but Henry only has himself once Lev graduates.
Finally, there's Coach Cox. He has only two rules: "(1) show up on time, and (2) don't get your hair cut the day before a game. Haircuts threw off a ballplayer's equilibrium" (16.19). (Good advice, coach.) Coach Cox also gives Mike money on one occasion, which Mike never pays back.