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
One day, Isa and Lea successfully snuck into the castle in order to investigate the mysterious cries coming from within. They found their way into the Heartless Manufactory and down into the Chamber of Repose, where they encountered a young girl known only as "Subject X". They befriended the girl and made plans to help her escape, but found that she had been moved elsewhere. In order to locate her, they decided to become apprentices to Ansem the Wise.

However, the very next day was the day that Terra-Xehanort turned on the other apprentices, transforming them into Nobodies in the Heartless Manufactory. Their strong hearts and wills allowed their bodies and souls to continue on as Nobodies. Organization XIII recruited the pair, naming them "Saïx" and "Axel" and inducting them in as Numbers VII and VIII, respectively. However, the two joined the Organization with the intention of taking it over in order to find their missing friend.

At some point during Sora's journey, he destroys Isa's Heartless.

Kingdom Hearts Re:coded
With the destruction of his Heartless and Nobody, Isa is revived as a human in Ansem'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
Near the end of Sora's journey, he destroys Isa's Heartless.

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.

Design
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, though he retains Saïx's X-shaped scar. 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 star and 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.