Winter is a stoat (or ermine, if you prefer). She was struck with ovarian cancer at a very young age, leading to the ovaries being surgically removed. Among other issues, hormonal imbalance has caused her body to interpret itself in a permanent stage of winter, so even in the most brutal parts of the summer, she has her thick white snow camouflage.
Alban, likewise, is a stoat (and very much prefers the short, hard-sounding word stoat to the soft and elegant ermine). He is also an albino - hair, skin, and eyes devoid of pigment, while he sheds his thick white winter coat in warm weather, it's replaced with a thin, lightweight, and similarly white coat. He wears sunglasses to shield his sensitive eyes (plus sunglasses pair really well with making corny puns).
Winter's motherly instincts were thwarted by her inability to conceive naturally. The pair resorted to enrolling in an experimental form of surrogacy, and specifically asked for the species of their baby to be kept secret from them - the guessing game of trying to figure it out was fun and exciting to them. As it turns out, their son was a mouse, and due to the mismatch between a mouse's very short gestation period of about three weeks and rapid development and a stoat's surprisingly long gestation period of about ten months, Colby was effectively in his late teens by the time he was born.
