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The devil is a deity, just not for Christians and some other religions.
Deities are not necessarily "good", as well. An Okami / Ninetail / Kumiho as example can be good, neutral, very evil up to some being which eats whatever sees him (or her).
"Gods" are not exactly referring to a level of power or influence, it's merely the category given by people believing in those (a few) specific ones. Others see their "gods" in as example Belial, Baphomet, Leviathan, Lucifer, Fenris or comparable.
At the end all are single instances out of a big pot filled with things that somehow differ from living beings. That's all..
Else the quite powerless wooden forest spirits wouldn't be in there as well.
But regarding contracts: there's just no need for one anymore. I am all for consensual and respectful bidirectionally positive outcomes, even without contracts.