Who is reaaaly X?(And some other revelations)

[Christmas]

Heart usually means soul,not mind.
But it can be an exception here.
The phrase means that shumis shift to the form of the ones possessing them(the ones on their mind).
If a shumi is not possessed they can become noble creature(moombas,etc...) if they have a good heart and a dark creature if they had a bad heart.

Why is there an exception? Cause you said so?

And "on our hearts" mean whether the heart is pure or defilled not some soul went into their heart and they just take the form of that soul.

So it is as good as saying if Laguna possessed a Shumi and Laguna is "on their heart" then the Shumi will evolve into Laguna.

And BTW, Zone look no where like Laguna.