Heartwarming Orphan

They're the characters the audience is predisposed to root for, suffering as they do from Parental Abandonment and in some cases, a tragic childhood.

They're the little orphan girls and boys who come into the story and proceed to melt the hearts of cranky old people and improve the lives of everyone around them just by being their cheerful, innocent orphan selves.

The girls may turn out to be little princesses and the boys might end up as Lords which that depends on the story.

This, of course, is their Happy Ending after a terrible childhood of drudgery most likely spent in an orphanage, on the streets or (shudder) under the care of their dear uncle or fond stepmother.

If they have foster parents or sometimes relatives, they can Earn Your Happy Ending by melting their cold and cantankerous hearts.

Heartwarming orphans don't have to do anything to be heartwarming. It seems inherent to their orphaned state.

They don't even have to appear on stage. Announce that an orphanage has been destroyed ans only a monster would do such a thing.

Examples
Nibbles from the actual cartoons was rather heartwarming at times, if a bit of a troublemaker.
 * Dick Grayson. Both Bruce and Alfred have commented that he brought joy and color back into their lives after he moved into Wayne Manor. Even as an adult, he's still universally loved by everyone who meets him, and still the best at softening Batman's heart.
 * Subverted big-time in Yu-Gi-Oh!: Kaiba, the orphan with the horrible past (his parents die, none of his relatives want to take him in, he gets continually bullied during his time at the Orphanage of Fear, he finally gets adopted by a megalomaniacal jackass who abuses him regularly and discarded his own son by blood for being too weak) ends up being one of the series' biggest Jerk Asses and his little brother Mokuba plays it straight though.
 * Lewis from Meet the Robinsons manages to win the hearts of the titular family, and gets invited to join them, over the course of the adventure and the invention fair.
 * In Despicable Me, we have the three girls Margo, Edith and Agnes being adopted by Gru.
 * The Kung Fu Panda franchise has Tai Lung, Tigress and Po. All were adorable kids and while Tai Lung later betrayed his foster father in his adulthood, Tigress and Po have done theirs proud.
 * Flynn's Back Story in Tangled, all the more effectively hinted at by his refusal to whine over it.
 * Flynn Rider: Ahh. . . yeah, well. I'll spare you the sob story of poor orphan Eugene Fitzherbert. It's a little bit of a. . . it's a little bit of a downer.
 * Koda from Brother Bear...especially after Kenai kills Koda's mother.
 * Lilo from Lilo & Stitch befriends a psychotic alien life form.
 * Little Robin Starling is such a heartwarming orphan that Tom and Jerry: The Movie abandons its lead characters and gives the movie over to her.
 * Anne-Marie from All Dogs Go to Heaven.
 * The Eskimo boy Kunac in Batman & Mr. Freeze: Sub-Zero.
 * Tarzan is a grown-up example
 * The title character of James and the Giant Peach, abused and neglected by his creepy aunts, is something of an Iron Woobie, nicely adapted in the movie version with the "My Name is James" song. Though miserable in his circumstances, he maintains inner strength, and acts as the leader of all his giant bug friends during their emigration to America.
 * Peter Parker was raised by his aunt and his uncle after his parents' death and Harry Osborn was orphan after his father's death.