The summoner class is very costly, They have the ability to summon any monster in the game, as long as they have the monsters summoning stone, which is droped by the monster itself at a lower rate. This also includes bosses and MVPs as well. Its really the only skill the summoner gets, however they have passive skills that increase your summon's abilities. The summon skills will act the same as if it were a normal monster they just follow you and attack everything that moves, though you can set their AI to only attack evil, or only attack what you attack, or even just kill everything.

Summons cannot be healed, but they will last and follow you as long as they are alive.

I have designed a summon point system, each summon takes a certain ammount of those and you can summon as many things as you want so long as you have points to back them up. Just know that the stronger the monster is, the more points it'll take.