Not too rare, but not that common either. I got a level 100 maybe five times out of the 60 or so gathers I did (assuming I did three gathers in the 20-something levels I got yesterday). I think they give about five levels or so on a level 20+ character.

Dunno about the girl in the tree line, but it's probably just a visual thing. Most people just skip the animation each time, and I doubt it changes the outcome since the items end up in your box as soon as you click the button anyway.

The rarest item you can is probably a Flower Gem. I only got one yesterday, but they're obviously valuable.

From what ive seen, the girl appear when you will get a rare chest.