Hmm..to be honest, I..like it!
But of course, no fanart is perfect--unless it's your own version/style of the character, or you just own--so here are some things I noticed that are either good or not, depending on the perspective:
It looks like the young version of Yakumo at first glance, and the similarities are very similar.
Her left eye looks a little strange, and it's a little less shaded for some reason than her right. Not trying to sound too insulting, but it kind of makes her look cross-eyed. To be honest, I like the left eye better than her darker eye, simply because it's closer to the manga.
Last of all, but not least, unless she's the young version, Yakumo would look great with a longer face and nose.
Her hair looks pretty awesome, and the shading looks great too. And I also like how you captured the shape of the face.
These are my opinions as a non-artist

but I hope they help. And I'm a fan of Yakumo because she's just awesome, like your picture

I want more!