When the party confronts Ouroboros at the Altar of Darkness when he addresses one of them he assumes their form.
At one point when he is talking about the Celestial Realm, he addresses Tiz and says that he sense a resident of the Celestial Realm in him.
After Ouroboros's defeat during the credits Tiz has Egil to go on ahead before he heads to the graveyard and says:
Tiz: I suppose i should return what i borrowed
Before a light leaves him. Tiz collapses suggesting that the light is the Celestial leaving him.
In the teaser footage for Bravery Second unlocked after the game, when Magnolia goes to get Tiz out of where ever he is, the way she talks is as if he is talking back to her:
Magnolia: Relax, There's no need to be suspicious of Me
pause
Magnolia: ...Yeh, I think that's fair to say...
Magnolia: I envy You Tiz
pause
Magnolia: Because, you've got the kind of good luck i'll never enjoy...
pause
Magnolia: ...You get to live in the same era as me, you lucky dog, you!
Magnolia: 'Kay, Let's get going.
Magnolia: ...To Where?
Magnolia: Mmm... to somewhere nice ♥
So this would indicate that Tiz only had a Celestial in him, he wasn't one himself which then left his mortal form (if he was he would be unresponsive to Magnolia)
I am wondering when did a Celestial enter Tiz? and did a Celestial enter every other Tizs' in all the worlds linked by Airy or was the Tiz we play as the only one?