Monster/Character boosts

Hey, so why is it that not all characters or monsters can be used as boosters? For example, you can have 4 of the same character, 1 can be used to boost, and the other can not be used? Is there something we can do to turn them to boosters? Level them up maybe?

You can only use monster/character/weapon for a boost of it already has a boost on it. You are actually transferring that boost from one monster to the other. Some are born with boosts some aren’t. You just have to get lucky. No amount of leveling up will provide a boost.