Our title character is a black-footed ferret. This lovable scamp enjoys ferret war dances; science-based webcomics like xkcd, Saturday Morning Breakfast Cereal, and Beatrice the Biologist; and curling up by the fire with a good book.
Being the title character, the Ferret in a Lab Coat first appeared back in comic #1 (If Donald Trump was a Scientist) and got those adorable ferret glasses in comic #4 (News Coverage).
Homer is a pedantic penguin poet. He's an English professor from Australia. If he was from New Zealand, they'd call him a "little blue penguin," but the Aussies refer to them as "fairy penguins," which is fine with Homer.
Homer's first appearance was in comic #7 (Haiku).
We wouldn't go so far as to call Cordelia evil. She just doesn't care. In fact, much of the time, she doesn't notice.
Cordelia's first appearance was in comic #11 (LD50).
Viktor is a bacteriophage (a type of virus that infects bacteria). He was "created" by Cordelia in comic #14 (Why the Long Phage?), but his first appearance was on Neil DeGrasse Tyson's tie in comic #6 (If Neil DeGrasse Tyson was a Microbiologist).