Margosa actually isn't talked about enough, IMO. The reason for that is basically she's really boring. All she does is sit there and kill things (most of the time in Beast Form, reducing how often you get to see her flashy skills). Don't let that distract you from how powerful she is, however. She's actually extremely tanky at level (especially in Beast Form, but that's obvious), being only second to Jigou. She also deals a ton of damage.
Again, though, she's really boring to use. I think the general reasoning is this: If you want a boring, OP hero, then Margosa is just Beresad lite.

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