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Easter Dave is a higher up in the criminal world. He holds sway over Miho Amakata and presumably many others. He bailed Nagisa, Haruka and Makoto out of jail when they were arrested for breaking and entering. Nagisa warns the others that they now owe him a favor. (This favor may have been paid by Rei taking care of his pet raccoon Mitzy, which was apparently something Rei had to do for him at some point.)
The only reason Rei eventually consents to doing favors is because it further enhances his cover. However, Mitzy can see right through, leading Mitzy to bite and give Rei rabies.
He has a long-standing rivalry with The Boss, though it is not explicitly shown in the show. He and Miho are criminal higher-ups of the same expertise, but their relationship is one of suspicion-driven comradery.
In the episode of Fairy Tail Abridged where his identity is revealed, he bails Nagisa out of jail after he is arrested for illegally breeding Rockhopper Penguins. Nagisa owes him lots of favors but Easter Dave still agrees to teach him magic.