Here is the full list of all 27 O Holy Knight achievements.
Survived First Wave
Won the game for the first time
Your have been wiped out for the first time
Unlocked Tristan
Unlocked Samara
Unlocked Moonshadow
Unlocked Elowen
Unlocked Gunter
Unlocked Titus
Unlocked Hemy
Unlocked Khan
Unlocked Lucas
Unlocked Hood
Unlocked Nethan
Unlocked Vespera
Unlocked Lucille
Unlocked Nox
Unlocked Riley
Unlocked Thorin
Unlocked Roland
Unlocked Aurelia
Unlocked Malachi
Unlocked Skyhawk
Unlocked Maximus
Unlocked Brynna
Unlocked Esme
Unlocked Luna