Because it stores several kinds of toxic gases in its body, it is prone to exploding without warning.
– Pokemon Red/Blue Pokedex
Koffing has a kind of derpy face, it looks really happy about something. For a Pokemon based on a ball of poisonous gas, it’s more amusing-looking than disgusting-looking.
Koffing has the ability Levitate, which is useful because it makes Koffing immune to its weakness to Ground, making it only weak to Psychic.
James had a Koffing in the anime, but it evolved into Weezing pretty soon.
Stat-wise, it has lots of Defense and decent Attack and Sp Attack.