Saix/Isa.

In their youth in Radiant Garden, Isa and Lea were close friends and sought to protect a mysterious girl, Subject X, from Xehanort's experiments. After losing their hearts, they became Nameless, Saix (Isa) and Axel (Lea), joining Organization XIII hoping to find information about Subject X. Saix rose to second-in-command, becoming cold and ambitious, while Axel formed bonds with Roxas and Xion, creating conflict with Saix. After being defeated and then reconstituted in the true Organization XIII, Saix finally regained his human form as Isa after Xehanort's defeat. Repentant, Isa reconciled with Lea, Roxas, and Xion, reuniting with them and keeping a scar as a reminder of his time in the Organization. His story explores corruption due to lack of heart, the power of friendship, redemption, and the search for identity.
Saïx/Isa is complex in Kingdom Hearts.
Saïx (Nobody) is cold, calculating, and ruthless, loyal to Xemnas but envious and bitter towards those with hearts, and cruel to others.
Isa (human) was serene, sarcastic, and very loyal to Lea, mature and calm. His motivation was to protect a lost friend, even as Saïx.
The transformation suppressed her kind side, intensifying her envy and creating Saïx's coldness, although remnants of her loyalty and past with Lea persisted in a twisted way.