Potamusu appeares after Igunisu's defeat. Actually she was the one to kill him, by hitting him by some kind of energy sword, saying "Oh, Igunisu, you ware my hero, but now you've changed so much, that I have to kill you, I'm sorry!".
      She was probably a water-demon, but I'm not sure, she haven't really fought herself, but always sent her servants. She already was abble to change outlook, so she masked herself as a regular student in Momoko's class. She also volontiered to be a manager for soccer team ( for those who don't know: in japanise schools, girls who are sport team managers aren't really what a manager is, they just help out and chear the players up ). Then she really seemd to fall in love with Yosukere and tried to make him love her too. However not too seccesful...
      During Potamusu's appearence Angel Salvia shows up the first time. Then in the last episode of her appearence Potamusu is already chased by Devilla's demon, course love made her no longer a demon. She tries to take whole school building with her to her world, but Devilla switches the way, so the building was supposed to fall in the Chaos-hall ( the one where Pluii died )... It appeared that Potamusu's love was true indeed, course she tried to save Yosukere even in front of demons, who ware sent to kill them. However love angels save them, and succes to take the school building out of the chaos-hall... That was the end of Potamusu's appearence as a demon, course she'd lost all of her demon-powers, she also recognised that love has to be from both sides and left Yosukere alone, since she already knew he loved Momoko. The end was rather funny, she saw one guy, and told him "Oh, you look like Igunisu" - "Who is that?" -he replied, - "No matter, - she answered -, let's just have a date".