Which Roxy also thinks about. We've also seen Roxy being patient, nurturing, and thoughtful with children. We've seen Eris do the same with older beastmen children in her own way.
The problem is that a minor scene, a footnote, or background character animation for Roxy or Eris is the entirety of Slyphie's personality.
Actual background characters and minor supporting characters have more character development than Slyphie does. Zenith has a more developed personality. Sylphie's entire narrative is already done unless things change when Rudeus drags Roxy into their household as wife number two.