Essentially, poor d’Eon occasionally gets possessed by the spirit of his dead sister, who magically transforms his body into her body, and uses it to fight zombies. (It’s more complicated, but that’s the gist). d’Eon goes from looking like this:

to this:

The transformation is completely uncontrolled by d’Eon (at least at first) and happens in the heat of battle. (It’s not clear from that choice of screencaps, but there does appear to be a physical transformation.)

Recently, a scanlation group released the first (over 90-page!) chapter of the Chevalier manga, and I saw comments that it was better than the anime, so I checked it out. It seems that it’s based independently on the same novel as the anime, and there are unsurprisingly differences. Rather large ones. The most immediately noticeable one is in d’Eon’s appearance(s):

Also notably, the possession is deliberately induced by d’Eon, and it is actually questionable (from a quick read) whether any physical transformation occurs. At the very least, the hair length difference is explicitly caused by d’Eon putting on a wig. In addition, pretty much the entire plot seems completely different, with at most vague similarities between the manga and the anime.

I really wonder which is closer to the novel, if the novel isn’t instead a third completely separate version of the same concept. (In the unlikely event that someone is curious, I like the anime version better than the manga version at the moment.)