It's getting ready to bloom, isn't it?
It looks a loot like a scutellaria...

Yall have native scutellarias over there, don't you?

It's neither of the two Scutellaria on this site ..

http://www.thewildflowersociet...

I would wait until it flowers, the leaves look a little like Viola. I have a lot of new self sets recently started to flower, they are small flowers on tallish plants which I get here and there from the wild Viola tricolor seemingly crossing with others I have grown years ago. Having said that, something about the form of your plant doesn't seem to fit.

I just noticed, it looks to have square stems which would place it in Lamiaceae.

I have Violas with square stems! The last photo of the leaf looks close to Viola, they vary a lot, but the first shot looks far from it and the 'texture' doesn't quite look right.