Alright so ive made a little guide on how to tell them apart

Mary - wears a locket, sports messy hair that obscures half of her face, has only one eye, is always drawn with the thousand-yard stare and bad posture

Loomy - wears a locket, hair is always neatly tied back, sometimes has an extra tuft sticking out, it depends on the occasion

Barbara - hair is always tied back, is drawn with more edge and a limp neck, sometimes sports dark circles under her eyes

Fikus - wears cargo pants and usually a locket, has a few strands of hair obscuring his left eye

Rubik - wears a locket, has short hair that's styled in a messy pixie cut