Is this character really original and not meant to be suzuki_jun from K-ON?

Doubt it. The artist has drawn k-on! before.

I leave the character indentification to you, because barely know anything about K-ON! :\

The artist doesn't give her a name, he says it's for practice. I think it's most likely an original.

He did not name post #737153's characters either, and that is without a doubt a k-on! image. I say go with our 'tag what you see' policy on this one.

I've seen K-On and K-On fanart. This is definitely Jun Suzuki. And a fine image of her at that.