Unique Satyr. Rhita can be recruited only if the Main Character has chosen Grass Affinity or the Elementalist profession. She can be found in a Town forest event, past day 10. She will request summoner training that take 2 hours, and then offers to join the team. Rhita also comes with a large Grass Cat.
Rhita can summon Grass Spirits with Grass Affinity like the Main Character, but they all spawn with the additional Berserker trait, encouraging you to raise them into aggressive roles. They're also a reliable source of Berserker to fuse to other units.
It's also worth noting that she comes with the powerful AncientPower trait, which can be Learned and Adapted to other characters that are level capped, giving them a large Stat boost while completely negating the annoying drawback.