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
On their way to Ansem the Wise's castle, Isa and his friend Lea encounter a boy named Ventus. Isa tells Lea they do not have time to talk to Ventus, but Lea tells him to "lighten up" and proceeds to challenge Ventus to a duel. Isa watches as Ventus defeats Lea; he playfully insults Lea's performance and then shares a laugh with the two boys. Isa reminds Lea that they have to go; the two friends look towards Ansem's castle, ready to proceed with their plan.

Some time after Terra-Xehanort is defeated by Aqua, Isa and Lea are thrown out of Ansem the Wise's castle by Dilan as Aeleus looks on. Following this incident, Lea buys two bars of sea-salt ice cream from Scrooge McDuck and gives one to Isa. Isa and Lea then 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 is consumed by darkness, turning him into a Heartless. Due to Isa's strong will, however, his body is able to live on in the form of a Nobody, Saïx. Saïx becomes Number VII in Organization XIII; he develops secret plans with Axel, Lea's Nobody, as well as the Organization's Number VIII.

Kingdom Hearts Re:coded
With the destruction of his Heartless and Nobody, Isa is revived as a human at the Heartless Manufactory, alongside the other former members of the Organization. Young Xehanort arrives and, with Braig's assistance, takes Isa in order to further Master Xehanort's plans to create a real Organization XIII.

Kingdom Hearts 3D: Dream Drop Distance
When Dilan, Even, Aeleus, Ienzo, and Lea awaken in Radiant Garden, Braig and Isa's absence causes them to think the two have not returned, which prompts Lea to venture out to search for them.

By the time Sora reaches The World That Never Was, Isa has once again become his Nobody, Saïx. When Lea attempts to retrieve Sora, he is attacked by Saïx, who he calls by his human name.

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 in coloration, 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 seems to be about the same age as Ventus. Isa's sky blue hair resembles his Nobody's in style, but it is shorter; Isa's hair is slightly longer than jaw-length and is spiked upward. He lacks Saïx's X-shaped scar, and he has sea green eyes, rather than yellow ones. His ears do not appear to be pointed like Saïx's, but he does have pierced ears.

Upon his revival in Radiant Garden as an adult, Isa appears to be physically identical to Saïx and even wears the black coat of Organization XIII.

Personality
Like his Nobody, Saïx, Isa is mature and has no interest in siding with Lea on childish matters. However, Isa has much more tolerance towards Lea, as he smiles while Lea and Ventus squabble. Isa values his friendship with Lea, but he is rather sarcastic and does not hesitate to lightheartedly mock him after he loses his fight with Ventus. Isa jokingly claims that he has tried to forget about Lea and his antics on multiple occasions.

According to Saïx, Isa is fond of dogs.

Trivia

 * "Isa" is a name in both   and.