[@Sep] Honestly the Dark Side wanderer serves little purpose to Byleth's overarching plot. His real purpose was to get Elda Beh out of the picture and Byleth to the temple. But he ended up providing some "angst" for young Tes as well. Internally, the Dark Side wanderer will probably play a minor role. Externally, unless you wanted to use him or something, I wasn't expecting him to make another appearance. 

Also the illness on Elda Beh was an experiment by the Dark Side wanderer.