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
Isa does not reappear in Radiant Garden following Saïx's defeat at Sora's hands, which prompts Lea to venture out to search for him and Braig. When Saïx attacks Lea in The World That Never Was on Master Xehanort's orders, Lea calls him by his former name in shock.
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.
Like his Nobody, Saïx, Isa is much quieter and more mature than his friend Lea, and has a similar manner of speaking. He has no interest in siding with Lea on a childish matter, but is shown smiling as Lea and Ven squabble. He has a carefree personality, and values his friendship with Lea, although he jokes about wishing he could forget it. He is rather sarcastic and doesn't hesitate to lightheartedly mock Lea after he loses his fight with Ventus.
According to his Nobody, Isa is fond of dogs.