The end of "Loser from the Earth's Core," when Nergal returns home after abandoning his family for half a year
Ms. Butterbean is legitimately touched when Billy gives her a gift as appreciation for being his teacher, even though said gift is a rancid apple she eats without bothering to wash and really look at and almost kills her.
Mindy's insulting Mandy in the episode My Fair Mandy where Mandy's entered a beauty contest against her. Grim, appalled by Mindy's behavior that actually seems to be hurting Mandy of all people, asks Mandy if she wants Mindy turned into a stinkbug. For all that Mandy's put him through, he's still willing to defend her against insults.