Similar behavior patterns, perhaps. Both are certainly very hyper/bubbly/chirpy/however you want to put it, but Selphie is far more mature than Rikku. Hyperactivity aside, her speech patterns indicate that she is not nearly as grown up as some of the other characters in other FF games. While Selphie may seem childish at times, she definitely has a more serious side that shows through. Rikku, especially in X-2, has no such offsetting personality traits.