An Anglicized spelling of the Welsh Rhys, this traditional boys' name got a big boost for girls in the early 2000s, thanks to the high-profile Ms. Witherspoon (who was actually born Laura Jeanne Reese, her second middle being her mother's maiden name). This spelling has been on the decline for boys since 2003, possibly for the same reason, but remains a strong, simple choice which might pose fewer spelling and pronunciation issues outside of the UK than the Welsh original.

Piper's rising popularity for girls makes it an unlikely masculine choice.

Willoughby is an energetic last-name-first route to the popular short form Will, livelier than any of the two-syllable options. It could be picked up by parents attracted to the Willow sound for girls.

With Willow and Willa becoming more popular for girls, surname-name Willoughby could be a more unusual alternative. Its full form sounds appropriate for both genders, but can be shortened for girls to Willow, Willa, or even Bee.