Chani Kynes is the daughter of Liet-Kynes, and she's also Paul's love interest. And, um, yeah, that's her character's purpose in a nutshell.
Chani provides Dune with its archetypal princess character. She's the daughter of the Fremen's leader, Liet-Kynes. She is often described as being "elfin," with youthful beauty (37.91), and her voice has "[s]uch feminine allure in [a] girl-child's voice" (34.177). In short, she's totally love interest material—if not marriage material (more on this in our "What's Up with the Ending?" section).
Okay, maybe that's not entirely fair. There are a few instances where Chani stands apart from the typical princess archetype. For starters, she kicks butt but doesn't bother taking names. In one scene, she dispatches a challenger in her lover's name simply so as not to "disturb [Paul's] meditations" (40.29). She also discovers how to awaken Paul from the near-death sleep brought on him by the Water of Life. It's a sort of reverse Sleeping Beauty kind of thing.
But these are exceptions and not the rule. In terms of plot and personality, Chani still remains the pretty-pretty princess character—though she could whoop any Disney princess twice over before breakfast.