Cluster Edge

This has nothing to do with demons but the style of animation is similar. One of the main characters goes to a prestigious school. The main cast from both series sing the opening and ending theme song too.  

Distant future, discrimination, action, experiments on human beings, bishounens as main characters - this is what you'll definitely like in "Cluster Edge" and in "№6" It will be to your taste. However, Cluster Edge is more similar to Gundam series (planes, robots), it concentrates more on the story-telling, not just fighting scenes.  

If you like 7-Ghosts, Allison & Lillia, Chrono Crusade, Fullmetal Alchemist, Kino's Journey, Kuroshitsuji, Pumpkin Scissors,Trinity Blood, or Valkyria Chronicles, you may well enjoy Cluster Edge as it is set in similar kind of alternate world and deals with questions about war, religion, spirituality, and what constitutes humanity 

It's mostly about a kid talented with airplanes and his/her adventures. As opposed to Allison & Lillia Cluster Edge shows some supernatural powers though. 

Superpowers, a single man persecuted by the military. Also the animation looks kind of similar. Plus both are those kind of series which you actually think are kind of ridiculous and overkill but at the same time you can't stop watching.