There are a few other characters in the book to mention. Kevin doesn't kill these people, but still ruins some of their lives, either directly or indirectly.
First is Kevin's BFF, Leonard "Lenny" Pugh, who reminds Eva of "brown smudges in underpants" (20.126). Gross. Lenny is a loser, which is why Kevin picks him as a friend. Hanging out with an invisible loser like Lenny is camouflage for Kevin. Lenny is also easy to manipulate, although sometimes he goes too far to get attention for himself.
One of these times Lenny goes too far is when Kevin falsely accuses drama teacher Vicki Pagorski of sexual assault. When Lenny tells the school board what Vicki did to him, it comes across as bragging. Franklin tells Kevin, "That friend of yours made you look like a liar" (25.186). Which is what Kevin is. Kevin is the one who went too far, accusing Ms. Pagorski of an assault she didn't commit. Franklin first makes light of it, saying, "This lady's got a taste for tenderloin" (25.7), but then he takes it seriously and wants to get Ms. Pagorski fired. As usual, Eva is the only one who can see through Kevin's lies.
Another person whose life Kevin tries to ruin is a girl whose name Eva doesn't know, so she calls her Alice. We'd call her Robyn, because she's dancing on her own. Kevin, who hates to see anyone enjoying him- or herself, whispers something to the poor girl. After that, Eva believes that Alice "had learned to never, ever take to an empty dance floor—possibly any dance floor—for the rest of her life" (20.137).
Mary Woolford is the mother of one of Kevin's victims. She deals with her grief by suing Eva, prompting Eva to say, "You can't imagine that you'll feel better if you win, do you?" (7.7). Mary has a right to be frustrated and angry, but turning it into a lawsuit is representative of a specifically American kind of behavior that Eva despises.
Eva and Franklin have two friends, Brian and Louise, who have kids before they do. The once hard-partying friends suddenly become "wholesome" (2.15). At least until Brian cheats and Louise divorces him. Not people you'd want to model your own relationship after.
Rounding up the rest, Eva consistently exasperates her defense lawyer, Harvey Landsdown, a man who probably finds Eva indefensible, not because of how terrible her son is, but because of how cold she seems on the outside. Eva watches a documentary by Jack Marlin called Bad Boy, which includes an interview with Kevin in which he—shockingly—seems to admire his mother. We're not sure if that's nice or one more way of Kevin manipulating everyone.
One of the few people Eva likes is Siobhan, the first person able to act as a nanny to Kevin without wanting to kill him. However, even Siobhan's not immune to Kevin's destructive behavior, and she tells Eva, "Sometimes they grow out of it. […] Sometimes they don't" (10.42). Kevin clearly does not. Eva also receives a rare moment of sympathy from Loretta Greenleaf, a fellow mother making a jail visit the same day Eva visits Kevin.
Finally, we have the rarely seen family members. Eva's mother, Sonya, is a shut-in who makes greeting cards. Eva sometimes talks about her estranged brother, Giles, but we never see him. Franklin's parents, Herb and Gladys, have the most parental sounding names ever, and Eva thinks they "have more in common with Kevin than anyone" (13.40) she knows. Why? Because they both abhor anything that doesn't have a specific utility or purpose. Eva wonders if she will ever see them again, after the death of her husband. We doubt it.