I still don't understand why Yukari's assistant (I don't want to say "slave") is so pure-hearted and innocent compared to her mistress.

FRien said:

I still don't understand why Yukari's assistant (I don't want to say "slave") is so pure-hearted and innocent compared to her mistress.

From a literary perspective, it's to provide a humorous counterbalance to her character, playing the part of the much-beleaguered straight-man to Yukari's more whimsical persona. In-universe, I'd guess that it's a trait she picked up due to Yukari's lengthy hibernation sessions when she needed to maintain the border essentially alone, in addition to trying to act as a positive role-model for her own shikigami.

HisRotundity said:

From a literary perspective, it's to provide a humorous counterbalance to her character, playing the part of the much-beleaguered straight-man to Yukari's more whimsical persona. In-universe, I'd guess that it's a trait she picked up due to Yukari's lengthy hibernation sessions when she needed to maintain the border essentially alone, in addition to trying to act as a positive role-model for her own shikigami.

I meant in-universe.
Kids are usually like their parents, because they copy what they think is "cool". And yet Ran, while not being the exact opposite of her mistress, is pure-hearted, diligent, and a bit naive.

FRien said:

I meant in-universe.
Kids are usually like their parents, because they copy what they think is "cool". And yet Ran, while not being the exact opposite of her mistress, is pure-hearted, diligent, and a bit naive.

I assume Ran was already a youkai fox when she met Yukari, which means she was closer to being an adult than an infant. Then Yukari added shikigami programming, which probably didn't include any commands for acting lazy or perverted. Sure, Ran could emulate Yukari's less admirable attributes if she wanted to, but she has two sources of better judgement.

FRien said:

I meant in-universe.
Kids are usually like their parents, because they copy what they think is "cool". And yet Ran, while not being the exact opposite of her mistress, is pure-hearted, diligent, and a bit naive.

"Pure-hearted" and "naive" depends on the author.

Anyway, talent, as they say, skips a generation. If someone has really diligent parents that are fastidiously tidy, then it means they can slack off, and never develop good cleaning habits themselves, as their parents take care of it. Conversely, their children, realizing that if they don't take care of things, then it won't get taken care of at all, will be more likely to start diligently cleaning. There's plenty of reason for a child to develop differing habits from their parents.