Yeah I've got a lot of ideas for this au in particular

So here's the rundown of who's what:

Piers- grim reaper. Was human before he died(not 100% how, probably protecting Marnie) but came back as a grim reaper. Instead of being a skeleton, he'd look more like Milo from monster camp, so almost black skin with off white skeletal markings over his body. Otherwise he'd look pretty much like himself

Gladion- living embodiment of the shadows or fear. Has monster and human form. In monster form he's just a shadowy figure with glowing eyes. As a human he looks much the same as his cannon self but shadowy looking hair and his eyes still glow

Megan- Dragon. She's my oc so- just look in my oc book to see what she looks like lmao. or better yet her toyhouse https://toyhou.se/11486708.megan

Phoenix- so I'd have to confirm this with AdminPhoenix (I'm gonna ping you a lot through this book I'm sorry) but we've agreed she'd be a Phoenix(a ice one, not the typical fire one). My personal idea is either a monster and human form, or one half form. If the half form, she'd look more like a harpy but more human. Bird legs, tail feathers, and arms that look like bird legs(think of a Kenku from dnd) with feathers running along the underside of her arms, most prominent at the elbows. No beak or anything, but some feathers in hair.

As for Marnie and bede:

Marnie- considering piers was human in life, Marnie is also a human.

Bede- now Phoenix has never agreed to this but she's never said no(probably because I've only brought it up once lmao) but I imagine bede being a fawn. If you want a example the first thing that comes to mind is elora from Spyro, but a more human looking top half.