Lea is the original persona of Axel, and a close friend of Isa, Saïx's original persona. He appears in Kingdom Hearts Birth by Sleep and Kingdom Hearts Birth by Sleep Final Mix as a teenager, alongside Isa, and then in Kingdom Hearts 3D: Dream Drop Distance as an adult.
Birth by Sleep
In the world Radiant Garden, Lea meets Ventus and befriends him quickly. Lea challenges Ventus to a friendly fight and loses. Lea leaves to sneak into the castle and bids him farewell and tells him to get their friendship memorized. Isa asks Lea why he instantly befriends people, which he explains that he wants everyone he knows to remember him, so he could live forever in the hearts of other people.
Between Birth by Sleep and Kingdom Hearts
Sometime after Ansem the Wise had taken Xehanort as his apprentice, he had plunged the entire world into darkness, turning everyone, including Lea, into a Heartless. As a Nobody, Lea found his way into Organization XII and received his new name, Axel.
Sometime after Sora's journey to save the world from darkness, he possibly comes across Lea's Heartless and destroys it, leaving only Axel to be destroyed.
Dream Drop Distance
Lea sees a dream of Axel and Roxas eating sea salt ice cream. Roxas reminds Axel of his promise that he'd always be there to bring them back and even uses his catch phrase 'Got it memorized?' and disappears. Lea finally awakens from his slumber and sees in the mirror the tear like marks under his eyes Axel has are no more and he finally realizes he and the other Nobodies that resided in Radiant Garden are human again. He understands Xehanort doesn't count and wonders what became of Braig and Isa and begins his search for them. Aeleus and Ienzo doubt they would find them believing if they were completed they would have been already found. Lea decides to use another method on finding them by using the Door to Darkness and comments 'Why do I always get stuck with the icky jobs?'.