Well, the manga quite clearly states that Cecil isn't actually sadistic per se towards Tia, he is simply calculative and warms up to her with time.
I presume it's because he has some actual development in character thanks to FL
It's kinda what the recent chapter is all about tbh
Oh yeah, that's also a very good point.
Cause of the character development, and we see it from his POV which helps us understand him
This! We could say unlike most shoujo isekai of the same theme, where the FL has much of the perspective, this manga has Cecil as the main narrator. He's practically the main character, not Bertia
Now that you mention it, that's also seems to be the case
He enjoys her when she's troubled he ain't sadistic (or is he?)
He does listen to her even thou he doubted her story (and stopped the pandemic right?)
not like shitty ml where they dismiss fl worries (he consoled her when crying at the Academy and was patient with her [the baby thing])
Never made contact with heroine for bertia (unwillingly did tho)
Was kinda possessive but never restricted her (didn't like her calling his aide brother though he let them get along)
You ve seen the archetype in many Japanese and Korean isekai novels before:
Cold, emotional unavaible, sadistic, perfect, superior, arrogant, bored and obsessive.
Basically a shitty tsundere guy.
Everyone who is aware this genre and trope also knows the royal aristrocratic ML/Alphahole.
So what makes him so different and more sympathetic than the other princes you know who has fun teasing/torturing the heroine/FL?