Chica

Before she became Chica, she was named Susie who attended a birthday party at Freddy Fazbear's Pizza. At the party, her puppy gotting hit by a car and Bonnie (William) telling her that he found her puppy, lured her away and killing her.

After that he lured four more children (Gabriel, Fritz, Jeremy, Cassidy) into a safe room where he brutally murdered them and hid their bloodied corpses inside soon-to-be-released animatronics, including Freddy Fazbear. The children's spirits remained trapped in the animatronics. When the animatronics were released, parents were quick to ask about blood and mucus oozing from their eyes and mouths; and deemed the animatronics too creepy and smelly, causing the restaurant to close down.

Soon the 'new and improved' Freddy Fazbear's Pizza was opened, starring new versions of Freddy and Co. based on the creepy animatronics, known as the Toy Animatronics. The old ones were kept for parts, and would come to life at night, with unknown reasons causing all the other animatronics to do the same (with the exception of the Puppet, who was seemingly constantly sentient throughout the day and night). Every night they attacked the night guards, hoping one of them would be the man that destroyed their lives.

Eventually this pizzeria was closed down after somebody used one of the 'yellow suits' once again for sinister purposes, and was replaced a few years later with yet another pizzeria of the same name, this time only using the original animatronics (with modifications to their designs to improve upon the original scary appearances). They still attacked the night guards, just as before, and their behavior continued until the pizzeria closed for good after its popularity reached an all-time low.

Years later, the original murderer returned to Freddy's, and destroyed the animatronics, thinking that would stop them from hunting him down. Instead, he unwittingly released the spirits of the first five children he murdered, who converged on him in the pizzeria's Safe Room. In terror, he hid in the same Spring Bonnie suit he had worn decades earlier. Despite thinking he was safe, the suit's age caused it to malfunction and kill him, allowing the spirits to leave the animatronic bodies. Eventually, the five spirits met once again alongside the spirit of the Puppet on the Happiest Day, where they were officially released from their animatronic personas and could finally be at peace.

Normal
In the first game, Chica appears as a tall, anthropomorphic yellowish duck, with teeth in her large orange beak.

She has two enormous eyebrows overlooking two dark and sinister eyes, which reflect her inner madness while she stares at the player through the window.

Age, severe decay, and attempted retrofitting has taken a toll on Chica's appearance in the second game, and made her even more monstrous looking.
She also lacks feminine features, making her supposed gender more androgynous. Like her original counterpart, she possess unusual features for a chicken, such as teeth in her beak, and violet eyes.

Chica's eye sockets are much larger than even the eyes themselves, and she doesn't have any eyelids or eyebrows, which make her stare much more horrific than before.

Her beak has been reshaped and turned into a jaw like Freddy and Bonnie have, which seems to be completely unhinged. It is also more pointed, like that of an actual chicken in reality and she has even more teeth on her upper and lower beak. The upper part of her mask has been dislodged, and pulled up, making her eyeballs sink in, her endoskeleton's teeth more visible, probably as she was supposed to be retrofitted but was left for spare parts and replaced with Toy Chica. The endoskeleton teeth is also clearly visible.

She has two stumps where her hands use to be, with long wires pouring out. She also seems to often have her arms raised and stretched out if she is "T" posed. But she doesn't do this while in the Right Air Vent, as she is forced to hold them together to fit into the vent.

The day, Chica appears as a joyful, harmless, nice, good-hearted and innocent robot; but the night, her persona turns into a considerably darker one's as she felt the urge to stuff the night guard into an endoskeleton suit. While chasing the player, she is a cunning and formidable opponent, albeit quite slow and considerably dimwitted as she make lots of noise each time she goes into the kitchen.

In the original game, she is the only enemy besides Freddy who attacks from the right, and can only be seen in the window. Chica is pretty rare and will not come at the door to The Office very often. Chica will only appear once or twice a night; sometimes two, rarely three. Chica will come in quicker than Bonnie, in about 5 seconds.

This is why she looks at the player through the window, though gives away the opportunity to her coming in by opening the door to see if she is there.

Like Bonnie, her killing of the player will occur if she's outside and the door isn't closed in time before it is broken. Otherwise, checking the camera will lead to a attack from Chica.

Chica can also disable the player's lights and doors. If this happens, she's already in the Office, hiding. Checking the cameras will trigger her animation and end the game. It is advised to close the door immediately if Chica is seen, while briefly checking the cameras for her.