@sketchy333 I think I get what you're saying and it's true it'd be nice to get some more backstory on Kaleid but I think if you reflect on what we've already learned about him you can see why he seems to have a double personality. Like you said, he's skilled at what he does, and it seems for some time he's had to play his role of fierce fighter. That constant fighting and being involved in war can easily make someone cold and hardened. Moreover, Kaleid is a special case due to his condition. He is naturally hypersensitive which makes even everyday sounds and background noise extremely tortuous. All his coping methods only help temporarily and only seem to take the edge off. It's been indicated that part of why he likes to kill is that he finds it alleviating - eliminating a source of that noise, plus the adrenaline rush likely helps to dull his senses a bit. Not to mention when everyone treats him like a monster, a part of him probably decided to play the role. In fact it wouldn't be surprising if the emperor encouraged him to play up that role to make him an effective enforcer. In addition, we have yet to see anyone that he's close to that would help him balance out or correct him.
As for why he's so different to Rinrin it's likely because she counters much of the above. She has both a mental and physical effect on him. Her behavior is different (thanks to her personality + past life) which intrigues him and her presence sedates him. She's also doesn't treat him like everyone else, initially due to ignorance but also because she finds him charismatic. We also see she's the only one daring enough to tell him off. There's the cliche but true adage too that love changes people. Keep in mind that this "second personality" only shows up for Rinrin.
As for Rinrin's bias, a couple factors come into play. First, it helps that she initially got to know him without knowing his identity. This allowed her to develop a bit of an infatuation before getting hit with fear. And while we have seen her be afraid of his killer side, she hasn't seen that side often so even if she knows that's the reality, it might not feel so real to her personally. Plus she has a practical personality. In her mind this is only for a year, the marriage is made (and she can't find a way to wiggle out of it) so she may as well focus on making the best of the situation. Most of all, Del is sweet and kind to her far beyond her expectations so it's probably hard for her to reconcile killer Kaleid with the Del she knows. I think there's a part of her that is still scared, but the side she's scared of isn't at the forefront of her experiences with him.
Sorry that was a bit long but just my thoughts 😅
