Here's my latest theory about who Angela is and what her role is:
After Sypha married Trevor, she left her role as a hunter of the church to raise her children. The church would soon grow to miss her though and sought out one of her kin to take her place. Angela was recruited and assigned to accompany Trevor on his latest mission to thwart the newly revived Count Dracula. Angela would become the ancestor of all the subsequent Belnades clan members who worked as witches under the employment of the Church, up to and including Yoko Belnades.
Ok, still don't know how Yoko managed to keep the last name of Belnades if that's the case though, but, whatever.