After playing many games with Karthus, there's something I finally realized about him...
Looking closely at his model, you can see something peculiar about his robes...
http://cdn.garenanow.com/webupdate/live/lolphweb/news/2014%20May/22/Karthus/IMG-Karthus-Models.png
It's difficult to make out, but after many hours of studying his model in-game and in Lolking's files, I finally realized a deep, dark secret about Karthus that we've never known before.
http://i.imgur.com/Fwb9ze9.png #HE HAS FEET!
This changes everything. Now we can discard the notion that Karthus is a floating dress, or a possessed mannequin. We can finally conclude, judging from his lore and everything about his motions, his magic, and his knowledge of elevated vocabulary and sweet songs of death...
...that Karthus is, in fact, not a flying broom man, but...
http://cdn.bulbagarden.net/upload/thumb/9/99/282Gardevoir.png/250px-282Gardevoir.png
#A GARDEVOIR!
And now, I rest my case, your honor. That is why, in fact, this man is innocent of mass genocide, manslaughter, and widespread destruction. http://vignette2.wikia.nocookie.net/aceattorney/images/f/f6/AJ_Judge_Surprised_1.gif/revision/latest?cb=20120828185106 Judge: ...
Gets up from defendant's chair "COWAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARDS!"
https://49.media.tumblr.com/8f5022ba4c0749864cf87122d5912bca/tumblr_nl9489yJri1sdk5d7o1_500.gif