Main and Secondary characters are two types of characters that you may need to be familiar with. The concept of these two aren't difficult to understand and using them is also a feat that is easily comprehensible.
--Nearly all of the information in this topic is taken from Support vs. Main:: Info and Limits, which was created by Kisuke and approved by myself.

~Main Characters

Main Characters are the characters that you must make when first starting out on this forum. Main characters use dice and have their own HP. When making these characters, more thought may need to be put into them and their techniques since they will be the deciding factors of how long your character will stay alive in the world. Also, Main characters are bound to their skill level by Tsukara, a force of energy within each warrior that is a limit to how many actions a character can use in a single battle since it decreases as different techniques are used. The Tsukara (Skill Bar) will grow based on how strong your character becomes, which will lead to your character being able to use more powerful attacks and being able to last longer in fights. Also, a Main Character has an experience bar, which will increase as they participate in battles and missions. Experience also allows a main character to grow and increase in skill. You must roll a specific dice for Main Characters to determine the outcome of their attacks, defenses, techniques, etc. You'll know the limit of Main Characters that you have by looking in your Battlefield and seeing how many spaces are still available.

Secondary Characters

Secondary characters are different from Main Characters. They do not use Dice Inventory, their attacks have no set attack power, and they do not have HP along with other things that Main characters have. Basically speaking, they are the original style of Role-Playing type character since you don't roll a dice for them but simply type the actions of your character and describe what's happening. Support characters, since they don't use dice, cannot participate in Tournaments where others have dice and they can't participate in missions, so there are a few downsides to Secondary Characters.