Blaggut. Nothing specifically that he did, just Blaggut's existence is the entire series' CMOH. He starts out as the typical Ugly CutePunch Clock Villain rat, patiently enduring his boss Slipp's abuse, then starts to get second thoughts when Slipp is contemptuous of his being nice to the lost baby mouse and mole they find. Then he cries his eyes out for hours when Slipp kills the badger mother. Slipp gets angry and starts to beat him up again, and Blaggut gets a Crowning Moment of Awesome/The Dog Bites Back moment by killing him and going back to the Abbey to face the music. And they exonerate him of all blame and he gets to be one of only two vermin in twenty books whose Mook Face Turn actually worked out. Awwwwwwwwwwww.
The water rats at the end of Marlfox. After the villains are defeated, they celebrate, flinging their weapons into the lake and becoming a peaceful farming community. They may have been the Mooks of the Marlfoxes, but it turns out that despite their apparent devotion the foxes have a 0% Approval Rating, so they have a happy ending too.
The end of Martin the Warrior is a Tear Jerker, even if it was inevitable *
since Rose obviously wasn't there for Mossflower
. Now, we know from the first book that Martin was Matthias. And Matthias and Cornflower got married. Then Word Of God said that just as Martin = Matthias, Rose = Cornflower. Cue the tears again...
The Freebooters get one for being the only vermin crew being actually attached to their leader (and he in turn for not leading solely through fear), suffering a BSOD when he's eaten by a snake instead of immediately squabbling over who's in charge.
In The Bellmaker, Silvamord forces the mute badger, Muta, to dance for her. The baby squirrel doesn't realize that they're trying to humiliate the badger, and merely sees it as a game, so he starts dancing with her. It ends up with the two laughing and having fun and Silvamord sitting embarrassed and furious that her joke backfired.
Another one from The Bellmaker. There's an otter in the book by the name of Rab Streambattle, who joined Muta in fighting off a horde of Urgan Nagru's grey rats while the queen and Truffen (Southsward's Prince) escaped. After both of them being horribly injured and Rab rendered mute, the final battle kicks in, with the Redwallers being let in by Rab's wife. She manages to spot him in the midst of the fighting and calls his name, prompting him to immediately start fighting his way to her, shouting "Iris! Iris!"