She's all cute and innocent until she gets into her giant mech.

Isn't her name Mirette & her "mecha's" name Pasvacan & collectively they are "code-named" Berserker?
That is what I'm led to believe anyways, hope I'm correct. :)

All I know its it's tagged f*te/mega and I have no clue what that is.

@Sci

If you look at the other f*te/mega images on Pixiv, they're by the same artist. Judging by the image titles and the tag s/he uses for them, it's his/her take on a Fate/Stay_night spin-off.

@Everyone else

No, that does not mean this image gets the Fate/Stay_night tag. The character designs are so far-removed to any resemblence to the actual F/SN chars that, without the the reference and context given by the Pixiv tag and the images titles, you'd never think to really associate this with F/SN at all.