I just know Sybilla is easily one of my best units, and she's made a lot of missions quite easy. Her DPS - ranged, no less - once her skill is active is nothing to sneeze at. The main thing for me is her range, which lets a more durable unit tank a tough foe while Sybilla wails away on it.
Been wondering that myself since I have two copies of her. From a look on the wiki, the comments on her page tend to compare her to a mix of Samurai and Monk, although her role is closer to that of a Samurai rather than a duelist-(ish) like a monk. Though I haven't found many concrete details the opinion of her is... less than stellar.