Eris age 8
Eris age 13
Eris age 15
(first kill around this age too, but this won't be shown until the OVA later)
Eris age 18...
Her development as a fighter is one of the key points of the series to me. Nothing is given to Eris, doesn't have any innate special powers, she just works hard and is driven to become great more so than anyone else. That sparring session with Ruijerd was meant to highlight just how far she's come from that bratty noble that who could barely lift a real sword above her head.
there's a lot of ramifications that come from turning point 2. Some immediate, others well down to line. Eris is one to watch moving forward as she's not one to take defeat lightly.