Isa

Isa is the human form of Saïx. In his youth, he was a close friend of Axel's human form, 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 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 are 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, known as Saïx. Saïx and Axel, Lea's Nobody, were both inducted as Numbers VII and VIII, respectively, into Organization XIII, but they secretly held an unknown plan.

Kingdom Hearts Re:coded
Following the destruction of his Heartless and Saïx, Isa is restored to his human adult form in Radiant Garden along with Ansem's apprentices sans Master Xehanort, albeit unconscious. He is implied to be the "poor soul" Braig decides to take along with him on his own agenda, as Young Xehanort says something that begins with an "S," possibly meaning Saïx.

Kingdom Hearts 3D: Dream Drop Distance
After failing to find Isa in Radiant Garden, Lea decides venture out to search for him and Braig. Later, when Lea arrives in The World That Never Was, he is briefly confronted by someone who appears to be Saïx, whom he calls Isa, revealed to be a Xehanort vessel.

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.

In his adult form following his resurrection, Isa closely resembles Saïx.

Personality
Like his Nobody, Saïx, Isa is mature and has no interest in siding with Lea on a childish matter, but has shown to have much more tolerance towards him, smiling as Lea and Ventus squabble. He values his friendship with Lea, but is rather sarcastic and doesn't hesitate to lightheartedly mock him after he loses his fight with Ventus, also joking that he has tried to forget about Lea and his antics.

According to his Nobody, Isa is fond of dogs.

Trivia

 * "Isa" is a name in both   and.