What happened to Lupin?
This is quite a downer of an answer, however there really is nothing more.

After escaping, within neither the anime nor manga, is Lupin mentioned again. I personally read Soul Eater and watched it, I don't recall any more sign of him, and this 'thesis' is backed up by the wiki:


  Debuts - 
  Manga - 'Death the Kid', 
  Anime - The Perfect Boy. Death the Kid’s Magnificent Mission?


As you can probably assume, this is indeed Episode 3, which I trust is the appearance you are referring to (being his only). It mustn't have ended with such a bad fate, as the use of the human souls, as we probably would have heard about a new Kishin being born.

I doubt it, as I'm not following it, however there is a possibility his name is at least mentioned in Soul-eater Not!, however I have yet to hear anything regarding that.