>>543190089
She's a max level paladin that buys him at a slave auction when she notices he is just tall enough to have an 8% increased encumbrance limit (due to not being a malnurished medieval peasant) and she can see that he has [character disapproves -1] messages, mistaking that for a sign of a plot hook. She then kills everyone, frees the surviving slaves but keeps him. The entire series is bickering about how alignment works (she thinks Evil is only for those who enjoy causing suffering and her opportunistic actions are Good or Neutral, he thinks callous disregard for others' suffering for gains is obviously Evil).
When he finally convinces her to become a genuine Good person, the series ends and the isekai goddess thanks him for saving that 1 soul she was keeping in purgatory for being objectively Neutral.