An Aka Manto is a particularly powerful and malevolent toilet-demon from Japanese folklore and urban legends who acts like a bogeyman, with innumerable stories told across many regions and as such the Aka Manto are said to come in myriad forms and possess any power available for the story.
As a general rule, however, an Aka Manto appears as a demonic figure in a red cape who manifests in a cursed bathroom stall within either public or school bathrooms, any who are foolish enough to enter this stall will be attacked and often the results are gruesome and inventive deaths - though some can survive.
Like many summoned spirits in urban legends there are no real benefits to bringing an Aka Manto into existence, since they tend to always murder or torture their summoner, this has not stopped the Aka Manto from amassing great popularity among people who enjoy playing such paranormal "games" (similar in many ways to the Western traditions of summoning Mirror Spirits such as Bloody Mary).