Aðalpersónan og aukapersónan eru góðar; persónusköpunin líka.

Because two singular nouns joined by og form a plural subject.

So:

• aðalpersónan = the main character
• aukapersónan = the supporting/secondary character

Together they mean the main character and the supporting character, so Icelandic uses the plural verb eru = are.

This works much like English:

• The boy and the girl are here
• Drengurinn og stúlkan eru hér

Góðar agrees with the subject.

In Icelandic, predicate adjectives usually match the subject in:

• gender
• number
• case

Here, both aðalpersóna and aukapersóna are feminine nouns. Since there are two of them joined by og, the subject is plural. As a subject, it is in the nominative.

So the adjective must be feminine plural nominative:

• masculine plural: góðir
• feminine plural: góðar
• neuter plural: góð

That is why góðar is the correct form here.

This is a very common question.

In Icelandic, adjectives used before a definite noun often take a weak form:

• góða persónan = the good character
• góðu persónurnar = the good characters

But after vera (to be), the adjective is in predicate position, and predicate adjectives normally use the strong form:

• Persónurnar eru góðar = The characters are good

So even though the subject is definite, góðar is correct here because it is a predicate adjective, not an attributive one.

They are the suffixed definite article, the Icelandic equivalent of the.

Instead of usually having a separate word like English the, Icelandic often attaches the article to the end of the noun:

• persóna = character/person

• persónan = the character

• persónusköpun = characterization / character creation
• persónusköpunin = the characterization

The exact article ending changes depending on the noun’s gender, number, and case.

Yes. Icelandic uses compounds very freely, and they are usually written as one word.

Here are the parts:

• aðal- = main, principal
• auka- = extra, secondary, supporting
• persóna = character/person
• sköpun = creation

So:

• aðalpersóna = main character
• aukapersóna = supporting/secondary character
• persónusköpun = characterization, literally something like character-creation

Then the definite article is added to the whole compound:

• aðalpersónan
• aukapersónan
• persónusköpunin

A useful rule: in Icelandic compounds, the last part is usually the core noun.

They are in the nominative.

Why?

• aðalpersónan og aukapersónan are the subject of eru
• persónusköpunin is the understood subject of the second part

Predicate adjectives after vera also stand in the nominative, so góðar is nominative plural feminine.

Because Icelandic can omit repeated words when the meaning is obvious from the previous clause.

The full version would be something like:

• Aðalpersónan og aukapersónan eru góðar; persónusköpunin er líka góð.

In the original sentence, the second part is shortened for style. The reader understands the missing idea from the first part.

This kind of ellipsis is common in reviews, comments, and concise writing.

No. It would be:

• persónusköpunin er líka góð

That is because persónusköpunin is singular, not plural.

So in the full version:

• persónusköpunin = singular feminine subject
• er = singular is
• góð = singular feminine good

The plural forms eru góðar belong only to the first subject, where two feminine singular nouns are joined together.

Líka means also, too, or as well.

In the shortened second part, putting líka at the end works much like English too:

• persónusköpunin líka = the characterization too / the characterization as well

In a full clause, you would very often see:

• Persónusköpunin er líka góð

So the final position here is natural because the clause is elliptical and compact.

The semicolon links two closely related parts while still showing a pause stronger than a comma.

Here it is useful because:

• the second part is closely connected to the first
• the second part is shortened and depends on the first for its full meaning

So the semicolon neatly signals: first complete statement; second related, abbreviated statement

A full stop would also be possible, but the semicolon makes the connection tighter.

Usually no. If the coordinated subject includes different genders, Icelandic normally defaults to masculine plural.

So:

• two feminine nouns -> góðar
• mixed genders -> usually góðir
• all/neuter plural -> often góð

In your sentence, both aðalpersóna and aukapersóna are feminine, so góðar is exactly right.