Catalyst characters in Genshin Impact are those who can deal Elemental DMG through their Normal Attack by default. This makes them an excellent candidate for Elemental reactions in battle, although there are some conditional downsides. For example, Klee won't be able to kill Pyro Slimes because they are immune to Pyro. This also applies to World Bosses, like the Pyro Hypostasis. Thus, to avoid this downside, it's always best to have various Elements at the party.