Well, it resembles an "h" if that's what you want to see. I've been searching for a little while, and the symbol on Subaru's forehead doesn't appear to match any Katakana symbol, nor does it match any Hebrew character, nor Astrological sign, nor Planet symbol, nor Greek letter, nor Cuneiform symbol, nor Phoenician letter, nor Cyrillic letter. I'll keep looking.
EDIT: Update! I think the symbol very closely resembles the Arabic letter "Jeem":
But it's not an exact match. I'll keep looking. *sigh*