The complete Catalog of TV Tropes, Idioms, and Devices. Plus, it’s a Wiki.

Example:

Jones the Cat Any utterly helpless or powerless victim whose primary purpose is to allow the audience to empathize while providing the opportunity for the hero to overcome their own fears and ultimately risk their life on that character’s behalf. Prominent in science fiction, and stereotyped by the Alien collection of films (Alien – The original feline Jones, Aliens – Newt, Alien Resurrection – Winona Ryder.) Willow fulfilled this role in Buffy the Vampire Slayer until she outgrew it (via forays into TeenGenius) and passed the mantle onto Dawn, the Buffyverse’s very own CousinOliver.