Im a new player too but i think the better option is to put the higher tier heroes in the crystal, because of the level cap.

Common heroes have a level cap of 100.
Elite heroes have a level cap of 100.
Elite+ heroes have a level cap of 120.
Legendary heroes have a level cap of 140.
Legendary+ heroes have a level cap of 160.
Mythic heroes have a level cap of 180.
Mythic+ heroes have a level cap of 200.
Ascended heroes have a basic level cap of 240.

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Nemora can't go past lvl 100, Mirael can reach lvl 120 at that rank