AZ's Floette is a Pokémon that AZ owned 3,000 years ago, when a war broke out between Kalos and an unnamed nation. Pokemon were drafted for said war, and AZ's Floette was one of them, where it died on the battlefield. Distraught, AZ would make a weapon that drained every living thing's life energy to transfer it all back to his Floette, which worked. An unintended consequence was that it made AZ and his Floette immortal.
Floette was so mortified that a genocide was committed to bring it back to life that it couldn't face it's trainer, and abandoned AZ. It wasn't until 3,000 years later when AZ learned from the player, Calem or Serena, the beauty of life, that Floette forgave and returned back to him.
In it's revived state, AZ's Floette has a special form called the Eternal Flower Form.