With all characters, cleared the game without continuing (Black Label or Black Novice mode only).
You have to clear the game with all 3 characters (Seseri, Ageha, Tateha) without continuing. Sounds difficult, but doing this in Black Novice is not a big challenge for regular shmup players.
Modes able to unlock this (confirmed): Black Novice
Modes able to unlock this (supposed): Black Label
Modes unable to unlock this (supposed): all other modes
* Changing setting in OPTION -> Game Config disables all achievements.
* Using Restart from pause menu disables all achievements in that play.