The chosen ones are reincarnated players, a relic from the reincarnation system where you got more stats from resetting your exp to 0.
Tiera is a chosen one, but she herself is abit..... special......
She have other powers that most people don't and that allowed the player reincarnating into Tiera to still be somewhat consious off the world around them... sometimes.
Kind of like being a ghost possessing a person but the person have no idea they are possessed.
( Tiera is only subconsciously aware of what happens and thinks she is just spacing out when she lose control, as it only lasts for 10 minutes at most )
But as a negative side effect she doesn't get the stat boost other chosen one get.
As you might have gussed by now that person is Marino.
She does manage to possess Tiera in a moment of lucidity when they visit Marino´s grave and tell Shin alot off stuff.
There is pretty much a full chapter of them just talking. (Vol 12 C3 )
a few of the things she mentions is however that her mind is getting more and more hazy and she is less and less lucid witch makes her think that she is merging with Tiara more and more.
Something that is backed up by the fact Tiera at one point magically got more stats and skills out of nowhere that Shin also at the point recognised as similar to his girlfriends skills.
At the end Marino useing Teira´s body tells Shin that it was becouse of her that Tiera came to "The Moon Shrine" as if Tiera dies so dose Marino, Schnee finding her was no coincidence .
The last thing she says is that she doesn't know what will happen if she fuses with Tiera making her a true chosen one and says goodbye to Shin.
this causes Shin to more or less break down for the next chapter but also gets him the push he needs to actually marry Schnee as he is no longer held back by Marino´s last words witch was to return to reality.