I've found a short plot summary (because the only thing I remember about it is the piano in the wood LOL):
A tranquil tale about two boys from very different upbringings. On one hand you have Kai, born as the son of a prostitute, who's been playing the abandoned piano in the forest near his home ever since he was young. And on the other you have Syuhei, practically breast-fed by the piano as the son of a family of prestigious pianists. Yet it is their common bond with the piano that eventually intertwines their paths in life.

Definitely sounds Gackt-ish I'd say.