Hebi

Unique Naga. Hebi is an unusual case. She lacks the Unique trait and doesn't have any recruitment events, normally only showing up in battles. You can use her while playing as Creation via her EmpowerHebi skill, though there are ways to get her for your party.

After the first mission, while at home, you will get an event calling you to an unstable portal. Once you reach the end, Hebi will suggest lowering your guard down for some power. Refusing will end the event and you will lose some morale, agreeing initiates combat where you will fight Hebi and some Tentacles after having your Mana drained. Winning gives you the skill EmpowerHebi which allows you to summon her in combat. The only other instance Hebi can be found is when you purify Creation in the final mission, where Hebi will pop up as a replacement boss.

To obtain Hebi as a unit, you need to find an instance where Hebi is in combat without the Unique trait to MindControl. You used to be able to Copy her with a Slime Girl but that's no longer the case as Hebi is a token and Copy doesn't work on token units.

As with any other summon her power depends on Creation's Magic, and with sufficient Magic Hebi can go way over level 20. You can view her as a second Creation, spawning Tentacles and quickly ramping up lust. Hebi's downsides are that she constantly takes damage every turn, which is mitigated by a large Health pool at higher levels; and that the skill to summon her reduces Creation's max morale, which is difficult to recover.