Pre-Reversed History
In Reversed, Celeste doesn't have a history before waking up as a PokeHuman in Pierre's soup shop.
However, in her original incarnation, she was a Pidgeot, living on an island that was being used as a HQ for a secret facility doing genetic experimentation on both people and Pokemon. She was a suspected widow, after her mate, Talon, had gone missing some time before and deep down, she believed that something had happened to him and blamed the humans and their strange buildings. She was left with three baby Pidgeys to raise on her own.
One day, her nest was attacked by a male Fearow. Whether he was seeking to expand his territory, and thought that a single mother Pidgeot and her chicks would be an easy target, or whether he was simply hungry and driven to desperate measures, due to the humans' interference on the isle, causing most of the Pokemon to go into hiding, Celeste could not say.
She did have something to say, however, about someone threatening her babies.
Seeming a different creature, she drove the Fearow from her nest, the pair battling high above the forest canopy. Their struggle took them some distance, as they dove and swooped, clawing and beating at one another with their powerful wings. Finally, claws tangling and wounds growing plentiful, the pair plummeted to the ground, landing inside the fenced-in exercise yard of the experimental facility. The Fearow, shaken, but otherwise unharmed, managed to take to the air once more. Celeste, however, could not give chase, as she'd broken her wing.
She was found by a young security guard with a kind heart going by the name of Hayden Rutherford. While he received a deep gash in his arm from the panicked mother bird for his kind efforts, he did manage to get her taken in and have her wing set so that it would heal. Of course, she also ended up trapped in the facility, now one of its experiments and unable to escape. For a long time, she was plagued with worry over what had become of her babies on the outside...