Sera

When the previous Hero, Serge Flore, invaded their castle, her father sealed her in a time-stasis spell and teleported her away to keep her safe, assigning Viktor to oversee and protect her. After Kelvin and his party defeated Viktor, Viktor entrusted her care to him, which is what leads to Sera joining Kelvin's party at the end of Vol 1 of the LN (and the end of Vol 2 of the manga).