I think Casca is Kushan which was a South Asian empire in real life and are darker skinned in the Berserk universe.
Miura planned fairly far ahead.
While its just a name Casca is apparently a Portuguese word.
"Casca = bark, crust, bread crust; skin, rind, peel; shell, hull, husk"
Which makes sense for the character even more now considering her current mental state.
Miura planned fairly far ahead.