As partner and mercenary Digimon level up, evolution makes them more powerful. When evolution has occurred, it is easier to win against an enemy as you gain more strength in HP. Partner Digimon evolve to Champion at Lv.11, Ultimate at Lv.25, Mega at Lv.41. Mercenary Digimon evolves at the same levels as partner Digimon, but there is an evolution limit depending on the class of DigiEgg. Evolution can occur until Champion when the class of DigiEgg is General, Ultimate when Advanced, Mega when Rare.
Examples of evolutions:
Gabumon Evolution Line (without ZeedGarurumon).