Learn Hardcore Finnish: Pidän päähenkilöstä, koska hän kasvaa tarinan aikana rohkeammaksi. - I like the main character because she/he becomes braver during the story.

päähenkilöä → partitive, would not work with pitää in the like sense
päähenkilön → genitive, would normally suggest possession (the main character’s …), not fit here

So you should memorize the pattern:

pitää + elative → Pitän kahvista. (I like coffee.)
Pitän päähenkilöstä. (I like the main character.)

What verb form is Pidän, and what are the main meanings of pitää?

Pidän is:

verb: pitää
person/number: 1st person singular
tense: present
meaning here: I like

So Pidän päähenkilöstä = I like the main character.

The verb pitää is very polysemous. The most common patterns are:

pitää jostakin = to like something
- Pidän musiikista. (I like music.)
minun pitää + infinitive = I must / I have to
- Minun pitää mennä. (I have to go.)
pitää + object = to hold / keep
- Pidän kirjaa kädessäni. (I’m holding a book in my hand.)
- Pidä ovi auki. (Keep the door open.)

So you must use the construction to know the meaning. In your sentence, because it’s pitää + elative, it clearly means “to like.”

Why is there a comma before koska in Pidän päähenkilöstä, koska…?

In Finnish, you always separate a main clause from a dependent clause (subordinate clause) with a comma.

koska is a conjunction meaning because that introduces a subordinate clause.

So:

Pidän päähenkilöstä, koska hän kasvaa…
- main clause: Pidän päähenkilöstä
- subordinate clause: koska hän kasvaa tarinan aikana rohkeammaksi

Rule of thumb:

Main clause + koska-clause → put a comma before koska.
Koska-clause + main clause → put a comma after the end of the koska-clause.

Examples:

Lähden nyt, koska olen väsynyt.
Koska olen väsynyt, lähden nyt.

Can I start the sentence with Koska? For example: Koska hän kasvaa tarinan aikana rohkeammaksi, pidän päähenkilöstä.

Yes, that word order is completely correct and natural:

Koska hän kasvaa tarinan aikana rohkeammaksi, pidän päähenkilöstä.
= Because he/she grows braver during the story, I like the main character.

When you start with the koska-clause:

you still need a comma at the end of that clause
the word order inside the clause stays normal: hän kasvaa …, not kasvaa hän ….

Both orders mean the same thing; changing the order just emphasizes different parts slightly:

Pidän päähenkilöstä, koska hän kasvaa… → focus a bit more on what you feel (you like them, reason follows).
Koska hän kasvaa…, pidän päähenkilöstä. → focus a bit more on the reason for liking them.

Why is it tarinan aikana and not tarina aikana?

Aikana is a postposition meaning “during”. Finnish postpositions normally require the genitive case on the noun before them.

So:

base noun: tarina (story)
genitive: tarinan
postposition: aikana

→ tarinan aikana = during the story / over the course of the story

Other common examples with aikana:

kesän aikana = during the summer
yön aikana = during the night

Using tarina aikana (without genitive) would be ungrammatical.

What does the ending -ksi in rohkeammaksi mean?

The -ksi ending marks the translative case. In this sentence it shows a change of state: “to become / to grow into something”.

With verbs like:

kasvaa (to grow)
tulla (to become)
muuttua (to change into)

you often use the translative to express the result:

Hän kasvaa rohkeammaksi. = He/She grows braver.
Hänestä tulee opettaja. = He/She becomes a teacher.
Vesi jäätyy jääksi. = Water freezes into ice.

So rohkeammaksi literally means “into (a state of being) braver”.

How is rohkeammaksi formed from rohkea?

Step by step:

Base adjective: rohkea (brave)
Comparative form: rohkeampi (braver)
- Pattern: rohkea → rohkeampi (add -mpi)
Put the comparative into translative singular (change-of-state form):
- rohkeampi → rohkeammaksi

Why rohkeampi → rohkeammaksi and not rohkeampiksi?

Comparative adjectives in oblique cases do consonant gradation: mp → mm.
So:
- nominative: rohkeampi
- genitive: rohkeamman
- translative: rohkeammaksi

Parallel example:

suuri (big) → suurempi (bigger) → suuremmaksi (into being bigger)

Could I say hän kasvaa rohkeampi instead of hän kasvaa rohkeammaksi?

No, hän kasvaa rohkeampi is ungrammatical.

After verbs that express becoming / changing, like kasvaa, tulla, muuttua, Finnish requires the translative case (-ksi) on the resulting state:

kasvaa rohkeammaksi (grow braver)
muuttua paremmaksi (change into something better)
tulla vanhemmaksi (become older)

If you use the plain comparative rohkeampi (nominative), you would normally use it with olla (to be), not with kasvaa:

Hän on rohkeampi. = He/She is braver.

Is rohkeammaksi an object of the verb kasvaa?

No. Rohkeammaksi is not a direct object; it’s a predicative complement (sometimes called a “result complement”).

It describes what the subject hän becomes as a result of the action kasvaa (to grow).
The translative case is used here to mark that resulting state.

So the structure is:

subject: hän
verb: kasvaa
complement (resulting state): rohkeammaksi

Compare:

Hän kasvaa nopeasti. (He/She grows quickly.) – adverb
Hän kasvaa pitkäksi. (He/She grows tall.) – translative complement
Hän kasvaa rohkeammaksi. (He/She grows braver.) – translative complement

There is no direct object in this clause.

What does hän tell me about gender? Does it mean “he” or “she”?

Hän is a gender-neutral 3rd person singular pronoun in Finnish.

It can mean he or she, depending entirely on context.
Finnish does not grammatically distinguish between male and female in pronouns.

So:

hän kasvaa tarinan aikana rohkeammaksi
can be translated as either
- he grows braver during the story or
- she grows braver during the story

In everyday spoken Finnish, people often use se instead of hän even for humans, but in standard written language you use hän for people.

Could the pronoun hän be left out, like … koska kasvaa tarinan aikana rohkeammaksi?

Finnish can drop subject pronouns when they are clearly understood from context, but in this sentence it’s not natural to omit hän.

Reasons:

The previous clause’s subject is minä (inside Pidän), and the new clause has a different subject (hän).
- If you drop hän, it becomes less clear that the subject has changed.
In a subordinate clause like this, especially in written language, keeping hän is the normal, clear choice.

So the natural written forms are:

Pidän päähenkilöstä, koska hän kasvaa tarinan aikana rohkeammaksi.
Koska hän kasvaa tarinan aikana rohkeammaksi, pidän päähenkilöstä.

Omitting hän here would sound either wrong or at least awkward.

Why are both verbs, Pidän and kasvaa, in the present tense if the story is already written and finished?

Finnish (like English) often uses the “narrative present” when talking about the contents of books, films, games, etc.

So you use the present tense to describe what happens in the story, regardless of when the story was created:

Pidän päähenkilöstä, koska hän kasvaa tarinan aikana rohkeammaksi.
= I like the main character because he/she grows braver during the story.

This is similar to English:

“In this novel, the main character grows braver over time.”

If you really wanted to put it into past tense (e.g. talking about your experience reading it), you could say:

Pidin päähenkilöstä, koska hän kasvoi tarinan aikana rohkeammaksi.
(I liked the main character because he/she grew braver during the story.)

But the present is standard when you’re summarizing or analyzing the story itself.

Could Pidän here also mean “I must”, like “I must [something] the main character”?

No, in this sentence Pidän can only mean “I like”.

The “must / have to” meaning of pitää requires a different construction:

minun pitää + infinitive
- Minun pitää lähteä. = I have to leave.
- Hänen pitää opiskella. = He/She must study.

You must include the possessive form (e.g. minun, sinun, hänen) plus the infinitive. Just Pidän alone never means “I must”.

Because your sentence has:

Pidän + elative (päähenkilöstä)

we know it’s the “like” meaning: I like the main character.