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 note since Rose obviously wasn't there for Mossflower.
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!"
This memory is a little blurry but in Pearls of Lutra there was a more anti-hero/noble demon vermin serving Ublaz and is clearly tempted to fight the monitor lizards she is forced to work with. The kidnapped Abbot calls her 'his child' much to her annoyance and once the rivalry with the lizards boils over into a mutual kill the abbot tries to comfort her and she smiles. "My child, yes I think I like that. Thank you, Father."
Her name was Romsca, and she was the closest Jacques ever came to replicating Blaggut. She had her rough edges and was definitely a proud pirate, but not once in the entire book was she okay with anything Ublaz did, or any of the actions taken toward the Redwallers. It's implied the whole fight began because Lask (the lead lizard) wanted to eat the Abbot.
From Taggerung, Mhera the toddler ottermaid being comforted after the loss of her father and newborn brother by the spirit of Martin the Warrior.
Later in that same book, Tagg/ Deyna is rushing through the Abbey and accidentally bumps into an otterwife. As he's running to prevent a disaster, he just places her to the side and dashes off, whereas she's left looking like she's just seen a ghost. It turns out he was her long-lost son, and that he looks almost exactly like his father. The subsequent reunion is incredibly heartwarming.
Shogg's death in Triss is both this and a Tearjerker. Having sacrificed himself to save her from a trio of adders, his final request is that she continue on and not cry for him.
Leatho comforting Tiria after she hears news of her friend Brinty's death in High Rhulain. He brings her a wooden doll carved like a mouse so that she has something to bury, as per an old otter tradition regarding those who are lost in battle.
In Marlfox simply the fact that Gawjo is alive and reunited with his long lost family, his son whom he hadn't seen since he was a dibbun, now a fully grown badass warrior. Though somewhat subverted by the fact that Brian didn't show a reunion scene between father and son. The last scene with Gawjo has him heading off to talk to him and the scene just ends there. Though not showing the reunion is kind of poetic in itself, letting the reader decide how it went.
Flinky and Crinktail's relationship in Loamhedge. They're not the only Happily Married pair of vermin in the series, but the subtle, affectionate way they interact (he calls her "me ould darlin'", she holds his paws after the group gets away safely) is a rare thing to behold in a universe like Redwall. And it's wonderful.