Knowle Rohe was a militairy soldier who appeared from season 16 until season 19.
Whatever nothing is known about Rohe's past, since nothing was originally revealed.
He was first seen as someone who had transformed in a militairy soldier and infiltrating within Hogwards. Rohe was also infected with the dark water, which made him terribly strong. He tried to kill Molly Weasley, but she hit him with a Depulso curse, which injured him, but he immediatly recovered. He was also responsible for the thing that Puck was accussed of killing Rohe, who actually was still alive due to he can survive anything.
Rohe killed Voldemort with Black Flyers during the season 19 premiere, and also tried to kill Harry and Guus, but the plant Parfoisi was somewhere close, which is fatal to militairy soldiers and killed him instantly.