Creatures are used as side characters in a sense as they have their own techniques and exist, generally, as a separate entity and being and, as such, possess their own attacks, techniques, and ability. Being a seperate being would usually simply mean another character, but creatures have different ways of being called upon and they also have restrictions placed on them as to what they can do and how they can perform abilities and techniques. For starters, Creatures are either bound by a turn bar or alternatively have their own health. Also, they are bound by the rule that all characters can only roll a maximum of two die (unless conditions have been met where they can do otherwise). This means that they share the dice you can roll and you must strategically decide whether you wish to have them use one, both, or none of your die at the cost of you attacking, defending, or using any other skill that requires a dice. Unlike a separate character, creatures don't require a skill bar, meaning that they can use techniques more easily and without the constriction of having to pay for using moves. As creatures are used more in battle the amount of turns they are used or possibly their maximum health may be enhanced. Generally, creatures are called to your aid either by means of capsule (the most common summoning), by summoning technique, or ritual (which may require a technique or even some item). Chants or oaths are also methods although they are less commonly used.

As a side note, a creature(s) health or turn amount will affect your character during fusion, but this will be explained in a future topic that's specifically about fusion.