Piano no Mori - The Perfect World of KAI by Isshiki Makoto
This is a SEINEN manga. You might want to read this and further your seinen collection. (I’m sorry; I can’t help jabbing on someone who went to lecture me about seinen and Hunter x Hunter a couple of weeks back.)
For Hunter x Hunter fans, I’m sure you’ll love this series too. Mind that this is not an action/adventure manga but the characterization here is superb. This manga also won a Grand Prize from 2008 [12th] Japan Media Arts Festival.