Based on her reaction after he defeats her - and isn't it a little silly for him to be a better sword fighter too? - I suspect that her back story is that she came from nothing and clawed her way to the top.
His mom or not,the moment she threw her child, she wasn't fit to be called his mom, and between his biological mother who threw him and the family who adopted him, im pretty sure the safety of his current family is what matter the most.
@kaikalaila
Wait Char? As in the wee baby girl in a different nation from her? How does that make any sense? That's some evil queen and Snow White level shit.
@Mojo
Exactly. Anyone that can be considered a threat to her power is attacked by her. Hell the only reason she hasn't killed her other kids or others in her employ is because they aren't as strong as her.
Though I'm guessing the only reason the MC didn't kill his birth mother is because it would throw the nation into chaos as nobles war with each other to take over the country. Then his dream of a slacker life evaporates.
@Mojo
Same nation, just a different region. She considers Char a threat because Char has the potential to be stronger than her. And Just like how she climbed her way up to a queen, Char can do it too. And Char's in a better position to do it since her father is the king's brother IIRC.