Helena Ravenclaw (The Grey Lady), The Bloody Baron, The Fat Friar, and Nearly Headless Nick (Sir Nicholas de Mimsy-Porpington).
When I was cleaning up among my submissions to deviantart I found a sketch from early 2015 of the Hogwarts house ghosts. Since I was very pleased with the design of those sketches I became inspired to do a finished drawing mased on those designs. The one that underwent the most changes from those sketches was Nearly Headless Nick.
This was so much fun to do! But I was trying a lot of different approaches to how to colour the piece and how to deal with their transparancy. Anyway, I feel like this is the best I can do at the moment.