Isa

Isa is the original persona of Saïx, and is a close brother of Axel's original persona, Lea. He appears in Kingdom Hearts Birth by Sleep as a teenager, alongside Lea.

Kingdom Hearts Birth by Sleep
When Ventus goes to Radiant Garden looking for Terra, he meets Isa and Lea. Lea challenges Ventus to a duel, while Isa watches them and smiles. After the battle, Lea calls Isa out for not trying to make up excuses for his loss and declares that he should've had Ventus for a best friend rather than him, causing Ventus and Isa to laugh. 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.