זאת הדמות הראשית בסרט.

זאת means this.

In this sentence, זאת is a feminine singular demonstrative, because it refers to הדמות (the character/figure), which is a feminine noun.

Yes, you can very often also say זו in modern Hebrew:

• זאת הדמות הראשית בסרט
• זו הדמות הראשית בסרט

Both are commonly understood as This is the main character in the movie.
For a beginner, the important thing is: both can function as this for a feminine noun.

Because דמות is feminine.

Hebrew words have grammatical gender, and words that refer back to them often have to match that gender.

• masculine this: זה
• feminine this: זאת / זו

So:

• זה הספר = This is the book → ספר is masculine
• זאת הדמות = This is the character → דמות is feminine

In the present tense, Hebrew usually does not use a separate word for is / am / are.

So the sentence is literally structured like:

• זאת = this
• הדמות הראשית = the main character
• בסרט = in the movie

So Hebrew says something like:

This the main character in the movie

but in natural English we translate it as:

This is the main character in the movie.

Because when a noun is definite in Hebrew, its adjective must also be definite.

• דמות = a character

• הדמות = the character

• ראשית = main / primary (feminine form)
• הראשית = the main

So:

• דמות ראשית = a main character
• הדמות הראשית = the main character

This is a very important Hebrew pattern:
definite noun + definite adjective

Because the adjective has to agree with דמות, which is feminine singular.

The adjective ראשי / ראשית means main / principal:

• masculine singular: ראשי
• feminine singular: ראשית

Since דמות is feminine singular, the correct form is:

הדמות הראשית

not:

הדמות הראשי

Because in Hebrew, adjectives normally come after the noun they describe.

So:

• דמות ראשית = literally character main
• הדמות הראשית = the main character

This is normal Hebrew word order.

Compare:

• ספר טוב = a good book
• הספר הטוב = the good book

So הדמות הראשית follows the same pattern.

בסרט means in the movie / in the film.

It is made from:

• ב־ = in
• סרט = movie / film

When ב־ combines with ה־ (the), Hebrew merges them. So ב + ה + סרט becomes בסרט.

That is why you do not see a separate word for the.

A useful thing to know: in unpointed modern Hebrew spelling, בסרט can look the same whether it means:

• in a movie
• in the movie

Context tells you which one is meant. In this sentence, the natural meaning is in the movie.

דמות can mean figure, image, or character, depending on context.

In a movie sentence like this, it means character.

So here:

• הדמות הראשית = the main character

This is a common word in literature, film, and TV when talking about fictional characters.

Yes. That would also be natural Hebrew.

Compare:

• זאת הדמות הראשית בסרט = This is the main character in the movie
• זאת הדמות הראשית של הסרט = This is the main character of the movie

Both are correct, but they are slightly different in nuance:

• בסרט focuses on the character in the movie
• של הסרט focuses on the character of the movie

In many everyday situations, the difference is small.

A common pronunciation is:

Zot ha-dmut ha-ra-shit ba-seret.

Word by word:

• זאת = zot
• הדמות = ha-dmut
• הראשית = ha-ra-shit
• בסרט = ba-seret (in this sentence, understood as in the movie)

So the full sentence is:

Zot ha-dmut ha-ra-shit ba-seret.

It is natural Hebrew.

A native speaker could absolutely say this when pointing at a screen, a poster, or a picture and identifying someone:

זאת הדמות הראשית בסרט.

It is short, clear, and idiomatic. The main beginner features it shows are also very typical of Hebrew:

• no present-tense is
• adjective after the noun
• gender agreement
• the preposition ב־ attached directly to the noun