Light and darkness are two sides of the same coin; without darkness, there is no light.
The Keyblade wielders’ great war over Kingdom Hearts was fought by defenders of the light, servants of darkness, those who sought to reconcile the two, and those motivated by nothing more than lust for power.
Terra regain his consciousness and memories for a moment while being the apprentice Xehanort (with the help of Kairi’s light, Aqua’s magic spell on Kairi, Eraqus’s influence, or something), and he’s protecting little Kairi when Radiant Garden being invaded by heartless. Then MX came and AX become his fucked up self again, splitting himself, and sent away Kairi to Destiny Island
Actually I’ve thought about this idea when DDD comes out. Idk it just my wishful thinking, we don’t see much of apprentice Xehanort on screen before he split himself