Isa

Isa is the original persona of Saïx, and was a close friend of Axel's original persona, Lea. He appears in Kingdom Hearts Birth by Sleep as a teenager, and returns as an adult in Kingdom Hearts 3D: Dream Drop Distance.

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 the Keyblade Graveyard, Lea and Isa's plan is revealed, as both were seen being thrown out of Ansem the Wise's 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, Isa's heart was consumed by darkness, turning him into a Heartless. However, his strong will gave him the ability to continue on as a Nobody, who was found by Organization XIII and given the name Saïx, inducted into its ranks as Number VII. It is known that Saïx joined alongside Axel, and the two had a secret, unknown plan.

Kingdom Hearts 3D: Dream Drop Distance
Following the demise of his Heartless and Nobody, Saïx, Isa is revived, but he retains his memories from his time as a Nobody, among them his hostility towards Lea. Thusly, he joins forces with Master Xehanort. After Sora is captured in The World That Never Was and Lea, Riku, Mickey, Donald, and Goofy come to his defense, Xigbar orders Isa to attack Lea, who reveals his face to his bewildered friend. Lea, however, manages to knock Isa away and regroup with his friends. Afterwards, Isa disappears along with Master Xehanort and the rest of the second Organization XIII.

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.

As an adult, Isa looks identical to Saïx, right down to the X-shaped scar on his face, and wears the Black Coat Saïx wore while in Organization XIII.

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.

But following his restoration, Isa apparently becomes as cold-hearted as Saïx, shown when he readily obeys Xigbar's order to attack Lea, who had come to Sora's defense against Master Xehanort.

Abilities
While Isa shows no combat abilities to speak of as a teenager, he is shown to be a powerful warrior in Kingdom Hearts 3D: Dream Drop Distance, wielding the Claymore he carried as Saïx in combat. It is unknown, however, if he still maintains his control over the element of Moon, though it is more than likely, since Lea is still able to use fire-based attacks after his time as Axel.

Trivia

 * "Isa" is a name in both   and.