So this will be mainly about Don't Starve characters in their inverted forms.
HERE WE GO!!!
Milson- has short, blonde hair, is not a scientist and doesn't grow a beard.
Millow- hates fire, takes more fire damage.
Molfgang- weaker with higher belly, less afraid of the dark.
Mendy- hates darkness, hits harder, has a brother (not here, but he's somewhere...)
MX-78- hates spoilage, hurt by lightning, charged by rain, can degrade with gears. This character is quite mysterious...
Mickerbottom- can eat spoiled food, can sleep, is illiterate, has a lower crafting level.
Moodie- is a hippy. Can't cut trees.
Mes- has better stats and starves slowly. Is very talkative.
Waxwell (Waxmell)- has high health, loses sanity quickly, has no stuff.
Mebber- is a pig. Pigs are friend.
Migfrid- loses stats after killing enemies (a tooth for a tooth), is weaker, has no items, is vegetarian.
P.S. I remember Bed Head Zed came up with some ideas on the chat, so credit for him.