Mammon is the Avatar of Greed and is the second oldest of the seven demon brothers.

Lucifer, Satan, and the other five princes of hell are students at the Royal Academy of Diavolo, where they train to rule the Demon World. From following humans to falling in love, their individual quirks land them in interesting situations every single day! Will their antics get them expelled, or will one of these infamous brothers come out on top? 
 
(Source: Funimation) 
 
Note: Episode 7 is an Audio Commentary by the VAs, and has an extended runtime of ~35 minutes as opposed to the standard 5 minute long episode.

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Special episode of Obey Me! released as a lead-up to the anime announcement of the ONA series.

As school bells ring, a little sheep wakes up at the Royal Academy of Diavolo in the demon world Devildom–where seven eccentric demon princes await. This wooly critter is actually a human exchange student…so why are they so fluffy and cute, and why can’t they remember anything? Now, they must figure out how to turn back into a human and regain their memories as princes named Lucifer, Leviathan, and more raise hell with their devilish hijinks!
 
(Source: Seven Seas Entertainment)

The second season of Obey Me! 

The seven princes are back for more shenanigans at the Royal Academy of Diavolo. Whether they’re failing their tests from missing MC so much, getting lost at sea, or taking a demonic baseball game too seriously, these boys just can’t keep it together! 
(Source: Crunchyroll)

The Obey Me! one-page comics that we released on Twitter are now being released together as "Obey Me! Misadventures"! Separate from the manga series releasing on Manga Plaza, these one-page comics take a look at the funny daily misadventures of the seven brothers. 
 
(Source: MangaPlaza)