User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-175.145.178.8-20120119134424/@comment-1011548-20120119155527

Perhaps Amatsu is only unlocked by specific conditions at HR150; perhaps you've got to complete all previous Quests, or all Arena quests with every weapon, or something.

Or, as Art said, the poor guy just isn't in the game.