In MixMaster Online, Henches and Cores are central to the gameplay. Here is what each term means:

Henches:
Henches are adventure companions obtained by defeating monsters. When a monster is defeated, it may leave a core. After you recover that core, the monster becomes a loyal Hench that fights beside you. You can carry up to three Henches at once; they level with your character and offer type-based strategic advantages (some types beat others).

Cores:
Cores are the nuclei left by defeated monsters and are essential to create and evolve Henches. Each core has unique traits, races, skills, and mix formulas. Mixing two cores creates a new, stronger core. With over 1,000 cores and many unknown combinations, hunting for the perfect core is a core part of the journey.

In short: Henches are battle partners; Cores are the materials you recover and mix to create stronger Henches.