Get all Battle Masteries to unlock the final chapter. Also, you can unlock two new characters for New Game Plus when you finish this chapter.
Unlockable
How to unlock
Final Chapter
Get all Battle Masteries
Unlock two characters for New Game Plus
Finish Final Chapter
New Game Plus
Beat the game. Also, upon finishing the game, one can increase difficulty 1-10; as well as add a special item that increases stats. Unused stat points also carry over.
Unlockable
How to unlock
Change difficulty
Beat the game.
New Game Plus
Beat the game.
Stat Boosting item
Beat the game.
Recruit Mech-Hisui
In Stage 47 of Haruna route, If Kohaku and Hisui have a combined kill count of 50, then Mech Hisui will join you.
Recruit Satsuki
You can get her at the end of stage 29 of Takumi's route. You have to kill her twice with Shiki. She appears in stages 5, 20 and 21.
Final Chapter...