1. It's been proven. Karane's disproportionate monster strength is tied to her mental state---ergo, her being a tsundere = monster strength. It's a pure example of mind over matter. Iku's dull eyes of unhappiness said it all. This is especially weighty coming from such a hardcore massochist like Iku.
2. I said it once, but it bears repeating. Hahari, without Rentarou serving as her morality chain, would already be in prison.
3. This manga's art does a DAMN GOOD JOB of hitting all the right emotional cues to really get us invested.
4. I rpersonally really like the deconstructing into reconstructing approach that this manga uses to build up its characters cuz it constantly forces the MC and supporting cast to work. Granted, the scenarios can oft tlmes be utterly batshit insane, but the very premise already set the tone that this was gonna be nuts. Normal just wouldn't work here.
5. Karane is actually one of my favorite characters, so seeing her get a dedicated character arc is awesome! Plus, it really is interesting to see a diehard trunsdere like her really rip into herself like that so much that she's willing to chemically alter who she is only to have Rentarou tell her that she doesn't have to.
Now, I get that some folks might take issue with Rentarou's actions... and no, I didn't read any comments before writing myself, I just have a good sense by now that some ppl might actually think he was being too heavyhanded or insensitive towards her or some nonsense like that.
If anything, it was the EXACT OPPOSITE.
Rentarou simply didn't want Karane to deny or hate who she was and that if she was going to make that change, it should be out of a genuine desire to change not motivated by anything else, least of all because she hates how she is.... or by using drugs to remove those undesired parts by force.
It's actually rather sweet when you have someone who accepts all that you are, good and bad, and like Rentarou... I'd prefer ppl not to feel that they 'have' to change and that being a mildly dishonest, often violent, cutely shy tsundere is perfectly okay. Your feelings will stillcome through loud n clear all the same.