Plum is the most mysterious, she represents "sorrow" but "longing" is more accurate as there's a lot of overlap between the two words in Korean.
She mostly seems to be a cartoon girl existing in a human world which may represent minus's (and my) impossible desire to have sex with cartoons.

Bak is a simple-minded girl who loves playing around, eating, joking and has a somewhat childish sense of humor and nature. Minus said he based her off a girl he knew when he was younger.

Yi is a strict office worker who is easily frustrated and stressed by her job. She works out her frustrations via wild dominating sex with her boyfriends/partners.

Kim is a nurse who enjoys taking care of her patients, has a loving personality, a bit of an exhibitionism streak and is pretty sexually liberated/free-spirited.

also the separate pics