The noun paladin means a champion of a cause. If you are a paladin for animal welfare, you probably spend your free time volunteering at the local shelter and take part in fundraisers that help keep it going.
Anyone who fights hard for something they believe in can be called a paladin, although the word has old-fashioned associations. While the French word paladin means warrior, it was used specifically starting in the late 1500s to mean "one of the 12 knights in attendance on Charlemagne." As time went on, its meaning grew to include any chivalrous or heroic person.