You can gain access to unlimited characters either by purchasing them in the store for $2.99 or by clearing Unlimited Mars mode using the character you want to become unlimited.
Abyss Mode CG
Gain access to the specified CG by completing the corresponding levels in Abyss mode, as indicated below.
Electric Damage - Level 1
Kagura Girls - Level 2
Kagura Setting - Level 3
Terumi Setting - Level 4
Kokone Gadget Plan #1 - Level 5
Kokone Gadget Plan #2 - Level 6
Izayoi - Level 7
Nu-13 - Level 8
Noel & Mu - Level 9
Jin & Ragna - Level 10
Level Infinity in Abyss Mode
Once you finish all ten standard levels in the game's Abyss mode, you'll gain access to Level Infinity. You'll encounter unlimited characters and a depth of 100,000.
Arcade Mode Rewards
When you clear Arcade mode with a given character, you will gain his or her endings, location tests and CGs in the Gallery.
Story Mode Rewards
Once you clear Story mode, you'll gain access to Kagura Mutsuki, plus the Highlander Battle mode.