Guardian of the Diamond Arrow

Joan Ilion is called the Fighting Queen for a very good reason; she's very good at it. She's employed her skills for years protecting the weak, especially if the weak happen to be children. So when she sees a child about to be kidnapped, she immediately jumps in...and lands in the middle of a war in a medieval universe. She's too old to be able to pass through the gate and see those who live on the other side, but that doesn't seem to matter. And things aren't what they appear to be, which means it could be a girl from the Flip Side who could end the war once and for all.