There are 4 Summons in South Park The Fractured But Whole. These summon someone during battle to help you out (heal your team or attack enemies). The first two are unmissable from the story (Moses + Classi). The other 2 you need to unlock through side quests (Jimbo and Ned + Gerald).

#1 - Moses (Automatically unlocked in a story mission, unmissable)

#2 - Classi (Automatically unlocked in a story mission, unmissable)

#3 - Ned and Jimbo (From Lost and Found Side Quest)

#4 - Gerald (From For Nippopolis Side Quest)

You can still get the missing two after the story. Nothing is missable. Simply beat the story for the first 2 summons and then do the side quests for the last 2. You don't have to use the summons, just unlock them for the trophy.