Мне нравятся те детективы, где важна не только тайна, но и характеры людей.

Russian нравиться works differently from English to like.

• In English, I am the subject: I like detective stories.
• In Russian with нравиться, the thing that is pleasing is the grammatical subject, and the person who experiences the feeling goes in the dative case.

So:

• мне = to me
• нравятся = are pleasing
• те детективы = the subject

A very literal breakdown is something like: Those detective stories are pleasing to me.

You could also say Я люблю..., but that means I love... and often sounds stronger or more direct than мне нравятся.

Because the subject is те детективы, which is plural.

Compare:

• Мне нравится этот детектив. = I like this detective story.
• Мне нравятся эти детективы. = I like these detective stories.

With нравиться, the verb agrees with the thing liked, not with мне.

Те means those, but here it often feels closer to those / the ones / that kind of.

So те детективы, где... means something like:

• those detective stories where...
• the detective stories in which...
• the kind of detective stories where...

If you remove те, the sentence is still possible:

• Мне нравятся детективы, где...

That is more general: I like detective stories where...

With те, the sentence sounds a bit more pointed or selective: I like those detective stories where...

Yes, где literally means where, but in Russian it is also very commonly used with books, films, stories, situations, and other contexts to mean where / in which.

So:

• детективы, где важна... = detective stories where ... is important
• natural English: detective stories in which ... is important

This is normal Russian.

A more formal or more literally structural version would be:

• те детективы, в которых важна...

But где is very natural and common here.

Also, the comma before где is there because где важна... is a subordinate relative clause.

Because важна is the short form of the adjective важный and is used as part of the predicate: is important.

Compare:

• важная тайна = an important mystery
  
  • long form adjective, directly describing the noun
• тайна важна = the mystery is important
  
  • short form adjective, used predicatively

In your sentence, the meaning is where not only the mystery is important..., so Russian uses the short form важна.

Because важна first agrees with the nearest noun, тайна, which is feminine singular.

The structure is:

• важна не только тайна, но и характеры людей

This is a very natural Russian pattern: the predicate comes first and agrees with the first noun, and then another element is added with но и.

So the sentence is basically built like:

• важна тайна
• then: не только тайна, но и характеры людей

You may also see plural agreement in other sentences when the speaker treats both parts as a combined plural idea, for example:

• Важны не только тайна, но и характеры людей.

That is also possible, but the given version with важна is normal and idiomatic.

It is the standard Russian pair for not only ..., but also ....

Here:

• не только тайна = not only the mystery
• но и характеры людей = but also the characters/personalities of the people

This construction links parallel elements. In this sentence, it links two things that are important in the detective stories:

• тайна
• характеры людей

The comma before но и is normally used in this construction.

Характеры is the plural of характер.

In this context, характер usually means:

• character
• personality
• temperament

So характеры людей means people’s characters/personalities.

The word людей is the genitive plural of люди and shows possession or relation:

• характеры людей = the characters of the people / people’s characters

This is a very common Russian noun + genitive pattern.

Not directly.

You would not normally say:

• те детективы, которые важна... ✗

If you want a version with который, you need a preposition:

• те детективы, в которых важна не только тайна, но и характеры людей

Why? Because the idea is really in which, not just which.

So:

• где = natural, everyday, very common
• в которых = more formal or more explicitly grammatical

Both can work, but где is the simpler and more natural choice here.