Willo is a ♀ female name.
Willo is a Frisian masculine name as a short form of names beginning with “Wil-“, like Willibald or Wilhelm. But is is as well a girl’s name as a short form of Willow.
Willo means “resolute protector” for boys (from Germanic “wil” = will/desire + “helm” protection/helmet) and “willow tree” for girls.