Yomi generally wears Japanese clothes and a headpiece. In Yomi's Dark/Light ultimate evolution s/he keeps the twintails and purple hair from his/her second evolution. In Yomi's Dark/Dark ultimate evolution s/he keeps the ponytail and black hair from his/her base form.
Adachi Yousuke, the artist who illustrated Yomi, stated in a tweet that D/L Yomi was meant to be male with a masculine image in mind and D/D Yomi was meant to be girl with a feminine image in mind, but there was no concrete decision on Yomi's gender.
So unless said/shown otherwise in an illustration, D/L Yomi should be tagged as a boy and D/D Yomi as a girl.
