She's been valuable in the past 2 world bosses. Yes she looks kinda freaky but she's a keeper.
Used on world boss for debuff extension, can also be used for poison instead of Eve /Salome. Although I haven't found her very useful without uncaps, I get more damage from any extra attacker.
The queen of the abyss.
Story-wise she should discuss the world outlook with Navi. One of them wouldn't make it through.