[quote=@AquaAzura]
So would people without memories be considered soulless; Like Roxas at the start of 358/2? ?What can a soul do within another's mind? Can they turn them into heartless and nobodies? or Can they take control of the body if they have the will power?
[/quote]

Roxas was an unusual case, and I don't fully understand everything about his deal. But in this world, yeah that's a good assumption. 

Yes! They can influence the light or dark(according to their own nature) in the hearts they inhabit. One could, if they were particularly dark and particularly strong, overpower a heart and make it a heartless. Though the person in question likely wasn't of strong enough will to produce a Nobody. 

They could only take full control if there is no other willpower controlling the being they inhabit, but they can influence others if they choose.