Here is the full list of all 51 OKAMI HD achievements. It takes around 40-50 hours to unlock all of the achievements on Windows.
Defeat Yami.
Board the Ark of Yamato.
Win the race against Ida and Hayate.
Win the race against Kai.
Defeat the twin demons, Lechku and Nechku.
Learn how to use Blizzard.
Defeat True Orochi.
Complete your Animal Tome.
Defeat Ninetails.
Win the final race against Tobi.
Learn how to use Thunderstorm.
Defeat Evil Rao.
Complete five digging mini games.
Meet Otohime.
Learn how to use Catwalk.
Defeat Blight.
Learn how to use Veil of Mist.
Defeat Orochi.
Learn how to use Inferno.
Enter the Moon Cave.
Defeat the Crimson Helm.
Learn how to use Galestorm.
Learn how to use Crescent.
Learn how to use Watersprout.
Defeat the imps that possessed Princess Fuse.
Exterminate the Spider Queen.
Learn how to use Vine.
Fight Waka.
Learn how to use Water Lily.
Learn how to use Cherry Bomb.
Defeat enemies inside of a Devil Gate.
Learn how to use Bloom.
Meet Susano.
Beat Hayabusa's turnip-digging record.
Return Kamiki Village back to normal.
Earn a Cherry Tree rank for each item in the Results screen.
Learn how to use Power Slash.
Learn how to use Rejuvenation.
Make your way to shore without being swallowed by the Water Dragon.
Increase your Solar Energy, Ink Pot, and Astral Pouch levels to maximum capacity.
Complete your Fish Tome.
Complete your Travel Guide.
Complete your Treasure Tome.
Defeat all monsters on the Wanted List.
Collect all Stray Beads.
Learn all god techniques to fill up your Technique Scroll.
Complete your Bestiary.
Earn a Cherry Tree rank for each item in the Total Results screen.
Defeat all enemies at all three Devil Gate trial caves.
Defeat the Kusa 5.
Unlock all achievements.