Lady Sue from Akira Kurosawa's Ran. And it frightens Hidetora. His armies killed her entire family after she was sent to him as a political hostage. Her response: becoming a nun and forgiving him. It would be so much easier for Hidetora if she just hated him.
Annie, the 1982 film, when she's in the orphanage ... she just keeps thinking about tomorrow! And her sunny attitude is contagious and transformative.
In Cannibal! The Musical, this is Played for Laughs with Swan. When his companions are complaining about having no food and being weary and lost, he suggests building a snowman via cheerful song and demonstration. Even after one of his annoyed companions shoots him in the head, he still has a smile on his face.
Mater in Cars 2 is subjected to Break the Cutie when he realizes that everyone else sees him as a laughingstock and a buffoon. Yet after an apologetic reassurance from his best friend Lightning McQueen, he soon returns to his regularly cheerful and optimistic disposition.
An odd subversion in Cecil B. Demented: Raven is a Satanist, yet remains upbeat and perky, even as her fellow Sprocket Holes are being picked off left and right. If one didn't know better, one might question her grip on reality.
The titular heroine from the Swedish film Du Är Inte Klok, Madicken ("You're out of your mind, Maggie!"), is this, much more then in Astrid Lindgren's books the film is based on.
The title character of Ed Wood is constantly upbeat and optimistic about his movies, declaring that every film take is "perfect" even when glaring errors pointed out to him. He constantly believes that his next film will be a smash hit despite the overwhelmingly evidence to the contrary and rarely lets his spirits dim. Tim Burton claimed he wanted to make the movie through Wood's eyes, showing his sincere love for movies despite his utter lack of talent. The darker parts of his life are glossed over due to this.
Hairspray: Tracy Turnblad is infectiously optimistic, happily championing the desegregation of the Corny Collins Show. Even though there are times that she has doubts that she can succeed, she never gives up, and in the end, most of the cast is with her. "You Can't Stop The Beat!"
Played for comedy in Monty Python's Life of Brian, in which a crucified man tells Brian to keep his spirits up and closes out the film with a jaunty musical number, "Always Look on the Bright Side of Life."
Nomi Malone in Showgirls is a more believable and seemingly more cynical version of this trope. Nomi carries a knife, appears street savvy and effectively confronts a truck driver for hitting on her. However, she is shown to be ultimately trusting, to her detriment, leaving her luggage in his car to be stolen. Throughout the film she is betrayed for her trust. The callous Las Vegas show director Tony Moss mysteriously refers to her more than once as looking like a "Pollyanna", the first time for no apparent reason, the second time possibly due to her dress (though this still gives us no clue as to why he would name her as such).
Vanellope of Wreck-It Ralph suffers a great deal of discrimination and out-right bullying for being a glitch. Despite being looked down on by the other racers and being forced to live in a hovel in a hidden area, she remains cheerily optimistic. The only times she falters are when somebody threatens her car, and thus her dream of racing.
Delmar from O Brother Where Art Thou is a male example. As Pete raves that now he won't get out of prison until he's 84 years old, Delmar blithely says, "Well, shoot. I'll only be 82."