There are six different types of Elements for Sidekicks:
Fire
Light
Nature
Shadow
Water
Prismatic
The Elements on the Sidekicks provide 50% bonus damage on certain bosses:
Monster King (Normal) is weak to Shadow
Mildred (Nature) is weak to Fire
Fluffy (Toxic) is weak to Nature
Monster Queen (Shadow) is weak to Light
Spike (Ice) is weak to Fire
Ice Queen (Ice) is weak to Fire
In Boss Raids, certain Boss enhancements will change the weakness type:
Frozen (Ice) is weak to Fire
Molten (Fire) is weak to Water
Toxic (Toxic) is weak to Nature
Dark (Shadow) is weak to Light
Blinding (Light) is weak to Shadow
of Doom (Shadow) is weak to Light
from Hell (Fire) is weak to Water
The Prismatic Element
is strong against all Elements, so it will provide a 50% bonus damage on all bosses.