@Degernase
"So then you're admitting that she's useless due to the circumstances? If they were just leisurely adventuring childhood friends, sure it'd be a dick move by MC, but seeing how he has a very important and time sensitive goal to achieve he has an excellent excuse for excluding her.
If MC has something crazy like 30.000 times the growth of a regular person (getting a class to lvl 20 takes "a lifetime", 365 x 80 = 29200, so would take an average person ~30.000 days to do what he did in 1.), she's practically just a burden even from day 2 and the gap will keep increasing for every day that passes."
SHe is no burden. You guys are failing to address the situation. She is being followed by someone, you literally can't just leave her behind. In a way she is just in as much trouble as the MCs sister. Leaving her behind would be a worse move than it currently is. And even with the different growth rates, it won't change the fact she can still grow. And since she has the blessing of the witch skill, she should also grow pretty fast as a mage, which makes your calculation impractical since we can't ascertain how fast would she grow to say that she'll trully be a burden.
And If we go by the "useless due to the circumstances" argument, then MC is equally as useless since both of them were equally taken down by the same tactic.
Guys stop looking only at results, the "means" are just as important. You can never say that she is useless because she was never given the chance to fight. If the captors were more ruthless, no matter how much stronger MC is, compared to them, he would've been killed. He only managed to recover and murder those human garbage because the criminals were arrogant enough to consider themselves victorious and left MC behind, thinking that he'd be devoured by monsters.
This is literally not a situation in which you can ascertain a person's uselfulness and ability.
It's the main reason why people despise dirty tactics, because they are ultra effective and disregard things like ability and experience. The only way to deal with them would be by predicting it's usage, or being literally immune to some of them for some reason.