Unlockables

Berserker Ring - Once you beat the game, load up your clear game file and start a new game on either Normal or Hard. Play the game as normal and get to the battle where a Ghoul says he has a Power Ring on. Upon winning, you'll also recieve the Berserker Ring, which allows you always fight in Berserk Mode without having the Solar Data be 7/8 or MAX.

Berserker Ring Beat the game once and load the data and play up to where you get your first Karma Ring

Hard Difficulty - Either by loading a clear save from the first Digital Devil Saga, or by completing Digital Devil Saga 2 you can select your difficulty of either Normal or Hard. Hard mode makes it harder to escape, (3/10ths the chance of normal mode) enemies hit you for 1.5 times more damage, enemies have a 1.5 times higher critical hit ratio, you hitting an enemie's weakness results in 2 times damage, and allows you to face an extra secret boss.

Unlocking Heat - Inorder to unlock the characater Heat, you got to make the right choices through out DDS 1 and 2.

Note that if you do choose to Unlock Heat in DDS2, Ronald would be replace

In DDS 1:

When Heat decide to fight you in Coordinate 136, pick the first choice.

After defeating Varon Omega at Brute's Base, pick the first choice

Beat the game, after the credit roll, save the game when it ask you to.

Start DDS2, when ask to load save from DDS1, load the clear
data.

In DDS 2:

At the EGG factory, pick the first choice.
After defeating Vitra, pick the first choice.