The Battle-Type ID is exceptionally important even among other IDs, for it serves as your tank and fire support. They run interference for you in battle, defeat enemies for you, and assist you in laying the smack in a downward direction.
All Battle-Type IDs start out as a Magmard, and can evolve down several paths from there. The Magmard is the second Innocent Devil you get in the game, so you'll have plenty of opportunities to evolve them over the course of the game. Some Battle Type IDs will be needed to access certain parts of the game, and any will be essential during most boss fights, so make sure you spend some time beefng them up!
As with all Innocent Devils, Battle IDs have HP based on Hearts, and their abilities consume Hearts when used. Being damaged also removes hearts, but don't be shy about using their abilities, battle types have massive amounts of hearts - especially at higher levels - and their ability costs are generally cheap for the amount of punch you get in return.
All Battle IDs are listed Below:
Magmard is the basic Battle ID, from which all other Battle-Type IDs come. He basically resembles a 7' tall golem made of molten and solid rock swirling together in a humanoid mass. A Row of crystals rises from the creature's back. Magmard is a quality fighter for his small size, and has a very useful skill for crowd-control. Magmard himself is somewhat lackluster as a fighter, though, so it's important to evolve him ASAP.
SKILLS: Aura Blast
MAGMARD EVOLUTION OPTIONS
Magmard-->Golem= 40 Sword/Special crystals
Magmard-->Speed-mail= 40 Spear/Axe/Knuckle crystals
Straight from his appearance in Castlevania: Circle of the Moon, Golem is a quality Battle-ID that can turn into either Iyeti (a final form), or Juggernaut (which in tuirn can become 2 other Final Forms). Golem gets 2 versions of the same skill, both called Hip Press. This is a very potent move that causes the ID to jump forward, slamming into the ground to cause damage to a substantial range. It sometimes paralyzes and deals heavy damage, making it even more effective than Aura Blast. The downside is that to get Hip Press Level 2, you must perform 100 chain attacks successfully with your Golem. All in All, a good Battle ID, one that will suit you quite well as you build your ID up.
SKILLS: Hip Press, Hip Press Level 2
GOLEM EVOLUTION OPTIONS
Golem-->Juggernaut= 70 Sword/Spear/Axe/Knuckle crystals
Golem-->Iyeti (final)= 90 Special crystals
Speed-mail looks like giant metal knight with a glowing crystal where his abdomen should be. He moves faster than any of the golem-derivitive Battle Type IDs. However, Speed-Mail pays for this increased agility with very low defense and HP compared to other Battle IDs. Heavenly Sword is extremely effective against slow-moving and large targets, capable of hitting from a great distance away. Speed-Mail evolves into two of the "Final" form battle IDs, Rasetz and Corpsey.
SKILLS: Heavenly Sword
SPEED-MAIL EVOLUTION OPTIONS
Speed-mail-->Corpsey (final)= 90 Sword/Spear/Knuckle crystals
Speed-mail-->Rasetz (final)= 90 Axe/Special crystals
Juggernaut looks... A bit different, plain and simple. He resembles a 12' humanoid with a single eye on his torso, gold-colored armor, and massively muscled arms. His skills are pretty good; Homing Eye can cause surprisingly heavy damage and is good for keeping back swarming enemies, whilst Brute Force Level 2 is needed to open a few doors. He can evolve into Ironside and Liquid Golem, the last "Final" evolutions.
SKILLS: Homing Eye, Brute Force Level 2
JUGGERNAUT EVOLUTION OPTIONS
Juggernaut-->Ironside (final)= 90 Axe/Special crystals
Juggernaut-->Liquid Golem (final)= 90 Sword/Spear/Knuckle crystals
Iyeti looks like a giant furry...thing, with a purple shirt, a medicine bag, and a crystal bracelet. He's one of the "final" Battle-type forms, and Iyeti is completely different from the other Battle IDs. He has unusual stat growth, to boot. There are other Battle IDs better in some catagory or another, but Iyeti is well-balanced despite his silly appearance. His two abilities are somewhat lackluster; Shoulder Ride lets you ride around on your Iyeti, plowing through enemies, whilst Ultra Scream stuns those nearb. Both are kind of overpowered by skills the Iyeti already knows - but you do need one his first ability to get a fantastic item though, so it's worth getting one regardle. Iyeti cannot evolve.
SKILLS: Shoulder Ride, Ultra Scream
Truly intimidating! Rasetz looks like an arrow-ridden skeletal samurai armed with a blood-stained rusty katana. Blue flames flicker around his body, and grimson light glows from his empty eye sockets. Though many newbs favor Corpsey, Rasetz is indeed the better ID, and by far the fastest among battle types. His skills are excellent; Grand Wave sends an energy wave along the ground that plows through multiple enemies, whilst Glow Soul sends a series of homing projectiles at the enemy. The latter is excellent in boss fights, as each can cause several hits. Rasetz cannot evolve.
SKILLS: Grand Wave, Glow Soul
Think of a gigantic suit of demonic-looking armor and you've pretty much described Corpsey. Armed with a large, barbed sword, Corpsey is quite similar to Rasetz. He deals more damage per hit and is more durable, but is nowhere near as fast or accurate as Rasetz is. His Grand Wave move is identical to Rasetz's, but his Bone Storm Move is a lot different, slamming an enemy with a barrage of bone-shards. This move isn't as powerful as some people think it is; It's Dark-Elemental and the only real upside is that Corpsey's invulnerable while doing it. Corpsey cannot evolve.
SKILLS: Grand Wave, Bone Storm
Ironside looks like a giant steam-powered robot. His primary attribute is that he bludgeons things to death. Ironside is far and away the hardest hitter of the battle IDs, but is also painfully slow compared to Rasetz and Corpsey. Nonetheless a top-notch ID and one that's worth using - even better, correctly leveled, he can be used for any area in the game that requires a specific ID Skill (due to Brute Force Level 2, with the exception of that one that requires Iyeti's Shoulder Ride). Chain Punch deals MASSIVE damage at long range, and Machine Gun Shot rakes a whole area with bullets that deal damage like Bone Storm from Corpsey - but with a much longer range and in a broader arc. Ironside cannot evolve.
SKILLS: Chain Punch, Machine Gun Shot
Liquid Golem looks like a crystal surrounded by a blob of mercury shaped like a humanoid. Many talk down Liquid Golem, but he's quite powerful - because he's squarely average stats-wise. He's not as slow as Ironside, and not as fragile as Rasetz or Corpsey, so he's an excellent balance. His skills seem lackluster at a glance, but have an unheard-of degree of power when analyzed; Mercury Sphere allows the Liquid Golem to spin around causing damage - made better by the fact that he becomes invulnerable when he does! Magama Mode allows him to charge an enemy head-on, burning them alive with his body - even better, it's the only element attack that a battle ID can learn (with the exception of Bone Storm). This makes Liquid Golem an oft-overlooked powerhouse. Liquid Golem cannot evolve.
SKILLS: Mercury Sphere, Magma Mode