HMMMM. the thing about this is that the devil's transformations are often aesthetically related to the character (neptune and water, for instance) but also related to that character's problems. i think plant guy is definitely a good option for prax, and maybe a fun design bit to fold in would be giving him a way to "connect" with people?

SO, here's my personal take on it: theme his general body around plants, green skin with maybe flowers/buds if you want a more "beautiful" take and mushrooms if you want a more "grody" one? BUT: add on the theming of "roots!" he grows and trails roots behind/around him, like tails or feelers, and when talking to people they could nestle up against their feet to create a "connection" between them :3c