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Frank's party wandered around the forest as they defeated monsters, he had already noticed that Star and Flare, Alexander and Gwendolyn's Starlight Hummingbird and Blazing Mouse respectively had already reached Level 8, and had gained plentiful of bonus stats, as they had a higher growth than other normal monsters due to being of a High-Grade Monster Egg purchased through the System's Monster Pet Market Shop.
Although there was a concept similar to leveling up for monsters in this world, it was more akin to self-strengthening through some sort of battle-based cultivation that monster could undergo as they battled, and their beast cores reacted to it.
However, thanks to the System, the monsters were able to learn EXP more easily and Level Up, directly acquiring instant power through Stat boosts with each level, they even gained some new Skills to use, so they were progressing quickly.
Check "The Protagnist's Sister Is Actually the Strongest"!