Devils
Devils are those humans who have met their end, but have decided to look at their human lives as prologue and have made their afterlives their main focus. They more often than not make a contract with an existing devil right before death. On the other end, one of the main goals of a devil is to recruit other humans to their ranks via contracts. Entering into the contract turns a normal human and a normal death into a new birth, or sorts, as a devil.
Contrary to popular opinion, devils are quite loyal and have a strong honor code which nearly pervades the devil population. They will not disobey their superiors, and possibly to a fault observe protocols. Those who are not honorable or break protocol are shunned from the devil community and are often killed without remorse.
Devils, most of them anyway, start doing more ancillary work, which means a lot of contracts which may seem... Tedious. Although that's not to say a beginner can't get into a serious arc via a contract. But the main reason for even doing contract runs is to hopefully get a human's soul. More souls you have the more powerful you are as a devil. Easy, right?
Speaking of power, a devil is given a chess piece which matches up with their most dominant characteristics: the strong silent type are give rooks, the mobile and cutthroat are given knights, the magic healers are given to bishops, the magic damagers are given the queen, and the ancillary or otherwise underwhelming are given the expendable pawn. Who's the king? The house leader. Nearly every group of devils is lead by a prestigious house name, take the Gremorys for example, who is vying for the throne of the Underworld. All the king's pieces are entered into that house upon their starting devil life. The power, then, of a member of the house is determined by how many pieces he/she is worth. If you'll remember your chess--which you really should to play this well--then you'll remember that a queen is worth 9 pieces, a rook 5, a knight and bishop 3, and a pawn one. A devil can possess more than one piece, except for the king and queen, and so the knight, rook, and bishop can be twice as value as previously listed. A pawn, however, can be worth up to eight pieces, depending on how many pieces were used when bringing back said pawn. To be clear, a devil may only have one type of piece inside them; so no knight/rooks or whatever.
To add to their arsenal, devils are capable of wielding demon swords. These are powerful blades which can cause massive destruction. Devils, like in the case of Issei, can also wield sacred gears in battle, but this is less likely. Don’t get caught with a holy sword, though, as it will likely kill you. Devils are extremely susceptible to holy things. Holy water and holy swords will do massive damage to any devil regardless of rank or power. Even making the sign of the cross will give a devil a headache. As mentioned before, each piece has their own strengths and weaknesses. Here’s a brief rundown of those.
Queen: High magic damage, somewhat clunky physical movements. Has a high amount of health because of it’s being worth the most, even more than all pawns combined, but low physical defenses can prove to be a fatal flaw in the queen’s diamond-esque construction. Otherwise, this is essentially a magic juggernaut, sporting high magic damage a high magic defence. Also hinted in the series to be quite maneuverable, in accordance with real chess. They are not as fast as a knight in close-quarters combat, but make up for it by covering a lot of ground in rating games. But remember, as powerful as they are there is only one of these aces. Don’t lose yours too quickly!
Rook: The second most valued piece on the field, the rook is a physical powerhouse with no magical capabilities to speak of. High physical damage and defense, but extremely vulnerable to magic attacks.
Knight:The knight is the lightning on the team. Darting around the battlefield at unbelievable speeds, the knight is hard enough to see let alone keep up with… Unless of course you’re another knight. Seeming to having moderate capabilities in offense and defense, the knight relies on racking up tons of combo damage to take out its opponents. The knight seems to lean toward the physical side of combat, but it is not as inept as the rook.
Bishop: The bishop is the white mage of the outfit. It heals its fallen comrades and has a fair amount of magical defence. It doesn’t have much in the way of offense, physical or magical, though, so make sure this guy stays off the main stage and you’ll be able to stay healthy for quite a while.
Pawn: The pawn is perhaps the most underestimated of the pieces. Sure it isn't very mobile and its offensive and defensive capabilities aren't very impressive, but a team can carry eight of these little warriors. That means eight possible traps--more than that but this isn’t chess theory 101--for your enemies! Plus, s single person, like Issei, can absorb all eight pieces upon being turned into a devil and become second only to the queen in base power. Use the pawn wisely and you will win the game.
King: The king is the leader of a house of demons, and as such is not only very powerful but usually has some sort of special move or ability which sets him/her apart from the rest of the bunch.