Learn Croatian: Divimo se njezinoj hrabrosti.

The verb diviti se (to admire) always requires its object to be in the Dative case. While in English you just admire something directly, in Croatian the object receiving the admiration must take the Dative.

Hrabrost (bravery) is a feminine noun, but it doesn't end in -a. How does its declension work here?

Hrabrost belongs to the feminine i-declension group, which consists of feminine nouns ending in a consonant. For these nouns, the Genitive, Dative, and Locative singular forms all take the ending -i. Since the verb needs the Dative case, hrabrost becomes hrabrosti.

Why does the pronoun njezinoj take the -oj ending if the noun hrabrosti ends in -i?

Adjectives and pronouns must match the noun's gender, number, and case, but they follow their own declension rules. The standard Dative singular ending for feminine adjectives and pronouns is -oj, regardless of whether the noun itself is an i-declension noun like hrabrost or a standard regular noun like žena.

Is it also correct to use njenoj instead of njezinoj?

Yes! Njen and njezin both mean her. Njezin is often preferred in formal standard Croatian, but njen (making the Dative form njenoj) is completely correct and very common in everyday speech.