So I never told people about the Summons in Reaper, so here is a bit about them. As you will see there is different categories where some are more powerful against others.
For each boss you defeat, you get a new summon. While summoning, all of the enemies will pause while you see an animation of what is going on. It will affect the entire screen. He can have one summon in use at a time. The player can go to the pause screen to select a different summon. Once the lower blue bar is filled up they can cast their summon. This uses up the entire summon(blue) bar. The bar can be filled up again by collecting souls.