Commanders are units you can use or summon in boss battles. During boss battles the commander icon will appear at the bottom-left of the screen and tapping it will summon the commander. The power of the commander can be increased by leveling up and upgrading skill. To level up a commander you must have the indicated number of fragments. Fragments are can be obtained from Adventure mode by tapping the 'How to get Fragment' option and playing the indicated stages.
There are 6 commanders that can be summoned when you have the required number of fragments, they are listed below in order from worst to best along with where you can get their fragments.
PROD
Kingdom
How to get fragment:
Forest of Dreams 1 (Normal, Hard, Hell)
GRAND RICK
Spirit
How to get fragment:
Forest of Dreams 1 (Normal, Hard, Hell)
KAISER KROWN
Beastman
How to get fragment:
Rekra Plains 2 (Normal, Hard, Hell)
HASUUM
Smolet
How to get fragment:
Smolet Valley 3 (Normal, Hard, Hell)
SIGRUNE
Undead
How to get fragment:
Silent Mountain 4 (Normal, Hard, Hell)
ELF
Roderim
How to get fragment:
Celestial Forest 5 (Normal, Hard, Hell)