Farley Wink's secretary Sawyer from Cats Don't Dance is a Broken Bird, having come to Hollywood with talent and enthusiasm, but was crushed by Hollywood's humanist hegemony. Likewise for many of the Funny Animals sitting morosely in Farley's office. Danny Cat arrives with the same enthusiasm, which ultimately reawakens their performer spirit, and they finish the film glowing in triumph and applause.
Violet Parr from Pixar's The Incredibles starts out massively introverted and socially miserable, too imploded to even speak to a boy she likes. After embracing her super side to thwart the villainous Syndrome, Violet emerges from her cocoon and starts making friends.
Queen Elsa of Frozen, first when she sings her iconic Let It Go solo, and for the first time in forever releases the energies pent up in her, and later after she truly masters her powers.
The titular kitten Oliver from Disney's Oliver & Company starts out as a giveaway kitten in a cardboard box. Evening rain, a sewer of doom, feral dogs ... life sucks for this kitten. By the end of the story, however, the kitten has a name and a home and Jenny Foxworth ... yeah, life is good for Oliver.
In Homestuck, when Aradia first enters the comic, she's completely unable to feel emotion because she's actually a ghost. She insists that she doesn't mind this state, but everyone else finds her depressing to talk with. Then she gets better. Able to feel emotion again, she chooses to look at the bright side of everything and becomes unrelentingly happy. So happy that she regularly creeps her friends out.
RWBYs Weiss Schnee goes from a grumpy, arrogant, slightly self-centered young lady to one that while still quite uppity, cares about her teammates' well-being, will joke around with her teammates (much to their dismay), and has no problem openly fawning over a Precious Puppy.
PJ from Goof Troop starts out depressed and hopeless in the pilot. In the main part of the series and the first movie he's mainly a world-weary Nervous Wreck with some despondent moments and relatively frequent contentment. By the end of the second movie, he's the happiest guy around. These changes occurred because of the positive influences of befriending Max and leaving home and finding a girlfriend.