How do i make an item that summons mobs as allies for a limited time?

So basically, I'm a huge fan of re:zero, I want to make an awesome mod about it with custom monster animations and everything, But the thing is, I want to make the system like re:zero witch's re:surrection, Where you have cards to summon certain characters as allies for a limited time by using your mana, But, how do i do that in mcreator? I already made Elsa Granhiert as a mob, With custom model and everything