Gem Girl

Gem girls have a difficult obtaining requirements, unlike other spirits she needs genetic hp stat to be 0. This becomes increasingly harder as time goes on, genes in portal grow and your MC Spawns stronger spirits if they have higher magic. If you are planning to use this character, its advised to evolve 2 of them from the start of the game, as at that time the RNG will be in your favor. Alternatively you'll need to recruit them in cave portals [final spirit evolutions are rare, you can clear an entire map without finding one] or hope one shows up in the tavern hire list.

However all this trouble for this unit is worth it. Gem girl is the strongest tank in the game, tied with spike seal, as well as second strongest mage and has the highest pool of mana. If the 0 hp gene is bypassed via breeding, her overall stats have no drawbacks.