אני חושבת שהגיבורה של הספר חכמה יותר מהגיבור, ולכן אני זוכרת את הדמות שלה טוב יותר.

Because חושבת is the feminine singular form of to think / thinking in the present tense.

Hebrew present-tense verbs usually agree with the subject’s gender and number. So:

• אני חושב = I think (said by a male speaker)
• אני חושבת = I think (said by a female speaker)

The same thing happens later with אני זוכרת.

For the same reason: Hebrew present-tense verbs agree with gender.

• אני זוכר = I remember (male speaker)
• אני זוכרת = I remember (female speaker)

English does not show this difference, but Hebrew does.

שה־ means that.

So:

• אני חושבת שהגיבורה... = I think that the heroine...

In Hebrew, שה־ is usually attached directly to the following word, not written separately.

Examples:

• אני יודע שהוא פה = I know that he is here
• היא אומרת שהספר טוב = She says that the book is good

Hebrew often expresses of with של.

So:

• הגיבורה של הספר = the heroine of the book
• literally: the heroine belonging to the book

This is a very common structure:

• הבית של דני = Danny’s house
• השם של העיר = the name of the city

There is also a more formal/compact structure called construct state (סמיכות), but של is extremely common and natural.

The prefix ה־ is the Hebrew definite article, meaning the.

So:

• גיבורה = a heroine / heroine
• הגיבורה = the heroine

Likewise:

• ספר = book
• הספר = the book
• גיבור = hero
• הגיבור = the hero
• דמות = figure / character
• הדמות = the figure / the character

They are the feminine and masculine forms of the noun:

• גיבור = hero (masculine)
• גיבורה = heroine (feminine)

Hebrew nouns often have grammatical gender, and many human-role words change form depending on whether the person is male or female.

In this sentence:

• הגיבורה = the heroine
• הגיבור = the hero

Because the adjective must agree with the noun it describes.

The noun is הגיבורה, which is feminine singular, so the adjective is also feminine singular:

• גיבור חכם = a smart hero
• גיבורה חכמה = a smart heroine

So:

• הגיבורה... חכמה = the heroine... is smart

This is the standard way to make comparisons in Hebrew:

• יותר = more
• מ־ / מה־ = than

So:

• חכמה יותר מהגיבור = smarter than the hero
• literally: smart more than-the-hero

The מ־ changes shape a little depending on pronunciation, and before הגיבור it appears as מהגיבור.

More examples:

• גדול יותר ממני = bigger than me
• מהיר יותר מהאוטובוס = faster than the bus

That is just the normal Hebrew comparative pattern.

In English:

• more intelligent
• smarter

In Hebrew:

• חכם יותר / חכמה יותר = smarter / more intelligent

So Hebrew usually uses:

• adjective + יותר

Examples:

• יפה יותר = more beautiful
• קשה יותר = harder
• טוב יותר = better

לכן means therefore / so / for that reason.

The prefix ו־ means and, so:

• ולכן = and therefore / and so

In this sentence it links the two parts:

• I think the heroine is smarter than the hero, and therefore I remember her character better.

Hebrew very often attaches ו־ directly to the next word.

Here את is the marker of a definite direct object. It does not mean you in this sentence.

Hebrew uses את before a direct object when that object is definite, for example when it has the, a possessive ending, or is a proper name.

So:

• אני זוכרת דמות = I remember a character
  no את, because it is indefinite
• אני זוכרת את הדמות = I remember the character
  את is required

In your sentence:

• את הדמות שלה = her character

This is one of the most important Hebrew grammar points for English speakers.

Yes, שלה means hers / her.

So:

• הדמות שלה = her character

In this sentence, שלה most naturally refers to הגיבורה — the heroine. So the speaker remembers the heroine’s character better.

Hebrew possessive words like this come after the noun:

• הספר שלה = her book
• החבר שלו = his friend
• הבית שלהם = their house

Here טוב יותר functions adverbially, meaning better in the sense of more well or to a greater degree.

The idea is:

• אני זוכרת ... טוב יותר = I remember ... better

This טוב יותר describes how the speaker remembers, not the noun דמות itself.

Compare:

• דמות טובה יותר = a better character
  here טובה describes the noun דמות
• אני זוכרת את הדמות טוב יותר = I remember the character better
  here טוב יותר describes the remembering

This can feel unusual to English speakers, but it is very natural in Hebrew.

Yes, it is very natural Hebrew.

The structure is roughly:

1. אני חושבת = I think
2. שהגיבורה של הספר חכמה יותר מהגיבור = that the heroine of the book is smarter than the hero
3. ולכן = and therefore
4. אני זוכרת את הדמות שלה טוב יותר = I remember her character better

Hebrew word order is often flexible, but this sentence uses a very standard and clear order. An English speaker can think of it as quite close to normal English structure, except for details like adjective agreement, possessives after the noun, and the object marker את.