Soldato
Would expect she based it on the style taught from Syrio Forel and then perfected it a bit with the faceless dudes. She did fight a bit with that other faceless wench.
Her fighting style is a unique mix of everything she's learnt from Syrio, The Hound and the FM plus both taking advantage of the element of surprise/being underestimated initially and not having armour so she's able to stay agile and nimble against slower opposition. She stays true to the water dancing she learnt as a kid, but took what she learnt from Sandor in terms of not using Needle like a big sword against armour where it'd be useless - it's all about stabby stabby in the unarmoured spots like the knees and neck (which she does to Brienne about 3 times before she goes full srs knight on her) - and the reactions/parrying she got from being blind and getting whooped by the waif constantly.
Also some people were moaning about Needle being able to withstand Brienne's sword but it was clearly a training sword with blunt edges and not Oathkeeper.