before entering the academy, Sera had a run in with the "heroine" of the story. The heroine and some others were hunting a divine animal in her family's territory, a snake. Sera saved the snake, but Roux felt compelled to join the heroine. Sera wanted to let him, but he fought off the urge (which he described as some kind of enchantment or charm magic) and they rushed back home. Sera and Roux strengthened their bond and Sera revealed she had her previous life's memories, as well as knowledge of events to come. It's around chapter 30 of the WN