Figurative expression "Это в его духе"

Question

Это в его духе. / Это в её духе. / Это в их духе.

I assume this is a figurative expression with the meaning of "it sounds like him/her/them".

(в = followed by the prepositional case)

(его/её/их = genitive case)

(духе = prepositional singular case)

My question is:

1. "их" is plural, so shouldn't you say "Это в их духах" with the prepositional plural case instead?

2. Why is it incorrect to say "Это в тебя духе" оr "Это в вас духе" to mean "that sounds like you"? Even though they share the same construction?

Answer 1

Это в моем,твоем, его, её, нашем, вашем, их духе.

It's an idiom. Meaning

It's my, your, his, her, our, their manner/way.

Since it's an idiom, the noun духе doesn't change, but it has a possessive pronoun. Меня, тебя aren't possessive pronouns, so you cannot use them.

Answer 2

1. It's incorrect because их isn't a demonstrative pronoun for дух, but a possessive pronoun of people to whom дух belongs so to speak. It's also incorrect because в smbd's духе, where дух is in singular form, is a fixed idiom.

Literally

Это в их духе - This is in their spirit

2. It's incorrect because the idiom requires possessive pronoun for дух, while тебя, вас etc. are inflected personal pronouns.

Possessive pronouns of the 3d person их/его/её (their/his/her) derive from Genitive of personal pronouns они/он/она, but as all other possessive pronouns answer the question чей? and not кого? and grammatically are similar to adjectives except that unlike the rest of them these 3 don't inflect to match the case of the noun they describe.

Thus в духе (чьём?) их/его/вашем.