Isa

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

Kingdom Hearts Birth by Sleep
At some point during Kingdom Hearts Birth by Sleep, Isa appears along side Lea and encounters Ventus. 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".

In the ending credits, Lea and Isa's plan is revealed, as both were seen being thrown out of the castle by Dilan (with Aeleus nearby). In the next scene, Lea buys sea-salt ice cream from Scrooge McDuck and gives one to Isa as they stare up into the sky.

Descent into Darkness
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 II. It is known that he joined alongside Axel, and the two had a secret, unknown plan.

Appearance
Isa wears a short-sleeved, navy blue jacket with a light blue stripe going along his shoulders to the ends of his sleeves. The jacket's collar is rather high and is sky blue, as are the ends of the sleeves and the bottom hem of the jacket. There is a yellow crescent moon symbol prominently displayed on the left breast of the jacket, referencing Saïx's attribute in Organization XIII.

Isa appears to wear a black undershirt, as well as white pants and black shoes with what appears to be a grayish trim on the bottom. He looks about the same age as Ventus. His hair resembles his Nobody's style, but shorter, Isa's being slightly longer than jaw-length. Like with Lea, Isa's hair is spiked slightly upward. He lacks Saïx's X-shaped scar, and has green eyes rather than yellow. His ears do not appear to be pointed like Saïx's either.

Personality
Like his Nobody Saïx, Isa is much quieter and more mature than his companion, Lea, with a similar speaking tone. He has no interest in fighting on Lea's side in a childish challenge, but is shown smiling as Lea and Ven fight. He is shown having a more carefree personality compared to his Nobody and values his friendship with Lea.

Trivia

 * "Isa" is a name in both Sanskrit  and Arabic.