In KH3, the concept of "souls" was briefly touched on during Roxas and Xion's Keyblade Graveyard scenes. When Xemnas is about to finish off Axel, Xion stops him and says "We need him alive, you know that." to which Xemnas replies "We need only his heart to forge the key. We do not need his soul."
This indicates that hearts and souls are separate things in KH lore, and that when a person dies, their soul is lost, but their heart is not.
I think the white robe Strelitzia is our first look at what happens to a person whose soul has been lost. So far in KH we've only seen what happens when a person becomes a heartless or nobody, but we've never actually seen what happens when someone is truly dead.
Nomura seems to have chosen Strelitzia to be the one to show us how souls function in KH.