The Civilian is a non-player character near the ruin of a building in West Ardougne. Unlike other citizens in the city, they cannot be attacked.
Players who have raised their kitten into a regular, overgrown, wily, lazy, or hell cat can sell their furry friend to the civilian for 100 death runes (200 death runes if player has received Ardougne Cloak 1). Note that you can sell the cat to any of the civilians in the open area in northwest West Ardougne where there are a lot of rats running around—it does not have to be just the one of the ones pictured. You may also, instead of going through the dialogue with the civilians, use your cat on one of them to immediately sell it. Note that you may want to be careful as you cannot get the cat back, so it is recommended to sell a cat which has no particular use anymore (e.g. Overgrown cat).