Pollyanna

If you're looking for the character type, go to The Pollyanna.

Pollyanna a novel by Eleanor Porter, is the first of a series of thirteen novels known as "The Glad Books", about an orphaned girl living with her aunt in the early 20th century. Young Pollyanna goes by a philosophy called "The Glad Game" where she finds something to be glad about in every situation. Combined with her sunny personality, her presence helps to reform her dismal town and, most effectively, her miserable aunt.

The novel was an instant success, warranting twelve sequels by different authors and passing the name "Pollyanna" itself into the vernacular to describe the archetype she embodies. It was adapted into Movies and TV series several times including a 1920 silent movie starring Mary Pickford, a 1986 Anime series as part of the World Masterpiece Theater series and perhaps most famously made into a film by Disney starring Hayley Mills.

After Porter abandoned the "Glad Books" series, it was taken over by first Harriet Lummis Smith, then Elizabeth Borton, Margaret Piper Chalmers and finally Virginia May Moffatt. Later books took Pollyanna into marriage, motherhood and war, not to mention living in places as disparate as a tenement in New York, a castle in Mexico, and Hollywood - playing "The Glad Game" and warming others' lives all along.

Tropes used in Pollyanna include:

Bitch Alert: Movie version: Angelica the maid.
Blithe Spirit: The first book revolves around the title character reforming her town and its inhabitants by teaching them her philosophy.
Break the Cutie: Well, the Universe seems to be trying... At the end, almost succeeding
Brick Joke: Movie: When Pollyanna and Jimmy are in Mr Prendegast's house he randomly feels Jimmy's hair and says "don't they ever cut your hair in that orphanage". This then shifts to a dramatic scene with Aunt Polly and about 20 minutes later we see Jimmy and Prendegast in the barber shop.
Can't Get Away with Nuthin': In the movie, Pollyanna falls off the roof and paralyzes herself while sneaking back into the house from a town gathering Aunt Polly forbid her from going to. Crosses over with Too Dumb to Live because she was safe but went out to fetch a prize she won at the fair only to fall off the roof.
Cheerful Child
Defrosting Ice Queen: Three main examples. There's Mrs Snow, the crabby old woman convinced she's ill and who is rude to everyone. Also, in the movie only, Angelica, the cynical maid in the house. And of course Aunt Polly herself. Pollyanna defrosts them all.
Despair Event Horizon: When this happens to Pollyanna, just about the whole town tries to help. She eventually gets past her Heroic BSOD.
Floating Head Syndrome: The Disney version's DVD case.
For Happiness: Pollyanna likes everyone and wants them to be happy. She seems to accomplish this goal without realizing the size of her role.
Foreshadowing / Harsher in Hindsight: Pollyanna's father taught her the Glad Game when a missionary barrel they received contained a pair of crutches instead of a much-wanted doll. He said she could be glad she didn't need to use them. Near the end of the book (and movie) she finds herself severely crippled, with warnings that she may never walk again....
Happily Adopted: Several individuals in the series.
Happy Ending: Well, duh.
Heartwarming Orphan: two in the first book.
I Want My Beloved to Be Happy: In Pollyanna Grows Up; for numerous characters.
Kissing Cousins: Not literally but, in the movie, Nancy lies saying her lover George is actually her cousin Frank. Pollyanna is not fooled when she sees them kissing.
Letting Her Hair Down: In the 1960 movie, Dr. Edmund Chilton says that Aunt Polly was so much nicer when she used to wear her hair down. After an emotional conversation with him, Aunt Polly goes upstairs and lets it down for a moment.
Licked by the Dog: Some people find Pollyanna's friendliness to be this, at least at first.
Plucky Girl
The Pollyanna: The Trope Namer.
Pre-Approved Sermon: In the 1960 Disney movie version
Stepford Smiler: While Pollyanna's cheerfulness is mostly genuine, she occasionally shows signs of struggling to maintain that cheerfulness, most notably when she cries while praying to her dead father about how hard it is to be glad all the time and when she gets crippled in an accident and learns that she may never walk again.
Teens Are Monsters: Subverted as hard as a trope can be. 12 year old Pollyanna is nothing like your average teenage girl, even your average teenager in an era before there was a concept of adolescence. The "worst" thing she ever does is sit and feel sorry for herself after shattering her legs.
Think Happy Thoughts: Pollyanna's "game" involves finding a bright side to even the saddest situations.
Tsundere: Mrs Snow and Mr Pendelton (book)/Pendegrast (movie) are harsh and cold initially but eventually become good friends with Pollyanna.