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
A serene and self-possessed boy who always has time for a quick quip at his best friend Lea's expense.

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 have 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 358/2 Days
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.

Kingdom Hearts 3D: Dream Drop Distance
Though Isa has yet to appear in person in promotional material, a February 2012 issue of Famitsu mentions that Lea is searching for him following the demise of their respective Nobodies.

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 sky blue hair resembles his Nobody's in style, but shorter; Isa's hair is slightly longer than jaw-length. Like with Lea, the back of Isa's hair is spiked upward. He lacks Saïx's X-shaped scar, and has sea green eyes rather than yellow ones. His ears do not appear to be pointed like Saïx's either, but he does have pierced ears.

Personality
Like his Nobody, Saïx, Isa is much quieter and more mature than his companion, Lea, with a similar way of speaking. 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. He also has a comical side, as he jokingly tries to forget about his friendship with Lea. He is rather sarcastic and doesn't hesitate to mock Lea after he loses his fight with Ventus, though he does so jokingly.

Trivia

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