Identification of a feudal era / demon protagonist manga
Found it: Shinobi no Kuni


  Synopsis (MyAnimeLIst)
  
  Based on a novel by Wada Ryou, "Shinobi no kuni" brings a story that blends history and fiction as it shows the struggles happening during the 16th century in Iga, a region famous by the Iga Shinobi, the ninjas from Iga.


The protagonist name is actually Mumon, not Momo as I said. Would be hard to find it with a wrong name.