I know I won't forget you. Believe me, I try all the time.
Isa to Lea
Isa is the original persona of Saïx and is currently known by this name; he was a close friend of Axel's original persona, Lea. He appears in Kingdom Hearts Birth by Sleep as a teenager, alongside Lea.
Like his Nobody Saïx, Isa is much quieter and more mature than his companion, Lea, with a similar speaking tone. He has no interest in fighting on Lea's side in a childish challenge, but is shown smiling as Lea and Ven fight. He is shown having a more carefree personality compared to his Nobody and values his friendship with Lea. He also has a comical side, as he jokingly tries to forget about his friendship with Lea. He is rather sarcastic and doesn't hesitate to mock Lea after he loses his fight with Ventus, though he does so jokingly. Isa values his friendships as he became Saïx again to locate a female friend that he and Lea made when sneaking into Radiant Garden's castle. He and Lea even became apprentices to Ansem the Wise in order to free their unknown friend. It is revealed in secret reports that this was one reason Saïx and Axel worked to have Saïx rise in the ranks of the Organization after Isa and Lea became Nobodies, which was to locate their missing friend.
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 other facial features, having marine-colored eyes and normal pierced ears.
After returning back to normal, Isa's appearance was similar to his Nobody self Saïx, retaining the cross-shaped scar on his face but losing his yellow eyes and pointed ears that symbolised his carrying of Xehanort's heart. He wears a black tracksuit and jeans.
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 this, 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 are seen being thrown out of the castle by Dilan, with Aeleus watching on. 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 II
At an unknown point in time, his heart was consumed by darkness, turning him into a Heartless. However, his strong will gave him the ability to continue on as a Nobody. Some time later, Organization XIII found his Nobody, who received the name Saïx and was inducted into their ranks as Number VII. He soon rose through the ranks to become Xemnas's second-in-command.
When Isa does not reappear in Radiant Garden, Lea sets out to search for him. Further on, when Lea arrives at Where Nothing Gathers in order to rescue a comatose Sora, he is attacked by a member of the Real Organization XIII. The member's hood falls from their head, and reveals Saïx, whom Lea refers to in shock as Isa. The two lock weapons for a short time, before Lea jumps away with the comatose Sora under his arm. Saïx then departs from the room with the remainder of the Thirteen Seekers Darkness.
As Saïx was defeated, Isa finally awakens as a complete person again. In the epilogue, he hands out Sea-salt ice cream to Xion, Roxas, and Lea on top of Twilight Tower.