Kingdom Hearts Birth by Sleep
When Ventus is sitting in the Central Square and spinning Terra's hand-me-down Wooden Keyblade, he loses grip and it clatters in front of Isa and Lea. Isa properly introduces himself, but tells Lea to continue on. However, Lea goes on to tease Ventus for playing with toy swords as he finds his frisbees much cooler. However, Isa simply takes Lea's hot-headedness in stride and watches as he gets beaten to a pulp by Ven. Despite Lea's attempts to get Isa to comfort him for losing, Isa simply states the obvious and goes on to explain multiple other explantions that meant the same thing. After Lea comments that he needs a new best friend and hopes Ven doesn't have a friend like Isa, Isa simply laughs with them. After they have a little conversation, Isa and Lea walk off and look up at the castle, apparently "ready".
Some time after Xehanort is defeated at Keyblade Graveyard, Lea and Isa's plan is revealed, as both were seen being thrown out of the castle by Dilan, as Aeleus watches. Later, Lea buys sea-salt ice cream from Scrooge McDuck and gives one to Isa as they stare up into the sky.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts II
At an unknown point in time, his heart was consumed by darkness, turning him into a Heartless. However, his strong will gave him the ability to continue on as a Nobody: Saïx. Some time later, Organization XIII found his Nobody, who received the name Saïx and was inducted into their ranks as Number VII. It is known that he joined alongside Axel, and the two had a secret, unknown plan.