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 mingle, but Lea tells him to "lighten up" and proceeds to challenge Ventus to an impromptu duel. Isa watches as Ventus defeats Lea before he playfully insults Lea's performance and then shares a laugh with the two boys. Then 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
Sometime later, Isa and Lea successfully snuck into the castle and found their way into the Heartless Manufactory. They descended the spiral stairway hidden beneath the castle and emerged into a dark hall lined with cages. Having second thoughts as they felt a presence, Isa and Lea wanted to run, but they were stopped and beckoned by the faint voice of a young girl. They were unable to see her clearly in the darkness but they went on to befriend her. After that day, Isa and Lea continued to sneak into the castle whenever they could to keep the girl company, but Lea wanted to do more and free the girl from her cell. Isa shared his sentiment so they infiltrated the castle one day to save the girl but she was gone. They searched for her during their subsequent visits but she was nowhere to be found, close to believing they might have imagined her. To find out what happened to her, Isa and Lea approached Ansem the Wise to become his newest apprentices.

After Ansem the Wise's disappearance, Isa and Lea were in the computer room when their hearts were consumed by darkness, turning them into Heartless. Due to their strong hearts and wills, however, their bodies were able to live on in the form of Nobodies, dubbed "Saïx" and "Axel". They were inducted into Organization XIII as Numbers VII and VIII respectively. Behind the scenes, however, the two secretly planned to take over the group and continue to search for their missing friend.

Kingdom Hearts Re:coded
With the destruction of his Heartless and Nobody, Isa is revived as a human in Amsem's lab, alongside 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 and Riku 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.

Kingdom Hearts III
After the defeat of Master Xehanort, Isa is restored to his original form. On the Clock Tower in Twilight Town, Isa, donning new clothes, offers Roxas, Xion, and Lea some sea-salt ice cream. He sits down beside Lea and the four are joined by Hayner, Pence, and Olette. During their time together, the Gummi Ship appears overhead to take everyone to Destiny Islands. He is seen playing frisbee with Lea and Ventus on the beach until it lands on Donald Duck's sand statue. Isa then joins everyone in watching Sora and Kairi share a moment before Sora fades away.

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 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 Lea and 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. After Xehanort's defeat, Isa is once again restored to his human form with his eyes and ears returned to their original state. He wears a long-sleeved, knee-length high-low black jacket with black pants. The jacket has a short front zipper on either side and a column of five buttons on a dark gray flap next to a long zipper down the middle. Isa's jacket, its sleeves, and his pants have gray camouflage patterning along the sides, though it is wider on the sides of the jacket. Like his attire in Birth by Sleep, Isa's jacket has a high collar, a different colored stripe along the shoulders and down the length of the sleeves with a gold crescent moon medallion on the left breast, and he wears black shoes with the gray camouflage as the trim along the bottom.

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.

In Secret Report 6, it's revealed that Isa is capable of compassion and selflessness as shown by his offering comfort to his and Lea's new mystery friend whenever they visited her, and his strong desire to save her from her imprisonment as Xehanort's lab rat. When she vanished, Isa's desire to find and save her is so strong that it remains in his adult years, even during his time as Saïx.

According to Saïx, Isa is fond of dogs.

Trivia

 * "Isa" is a name in both   and.