Pretty much everything relating to Oswald and Gwendolyn is simply heartwrenchingly sweet. Most notably, when Gwendolyn discovers the ring Oswald gave her as a wedding gift (that she'd given to her father) has been stolen by the fairies and she single-handedly storms into their forest to reclaim it- not so she can return it to Odin again, but because It Was a Gift. Then when she discovers Oswald had been slain by a Halja as his despair at (apparently) not receiving Gwendolyn's love had sapped him of his strength and will to fight, she goes one better by single-handed storming the Underworld and slaying the Queen of the Dead to rescue him!
On Oswald's side of the story, when he goes on a full-blown quest to find an alternative for Gwendolyn being forced to love him by her father's spell because, more than anything, he wants her to love him for who he is.
Mercedes finally keeping her reluctant promise to the frog - a kiss, something that had Squicked her out before - as he lies dying after saving her life. And not only does it save him from death, he turns back into Ingway.
Even moreso, the extreme lengths the frog goes to in order to help Mercedes, culminating in nearly killing himself casting the spell that saves her from Beldor, and urging her to leave him behind afterwards rather than sparing any thought for the kiss he'd previously been trying to beg from her. Especially given that it proves how genuinely selfless Ingway is actually capable of being.
The final true ending of the game when you get 100% completion where Velvet and Cornelius are FINALLY freed from the Pooka curse to live the remainder of their human lives together.
Also, the World Tree growing where Mercedes died. Anyone who knows what that means will realize now that Mercedes' dying sentiments were wrong: she wasn't useless.