Learn Portuguese (Portugal): Aquele transeunte está a ler um livro e parece ser muito reservado. - That passerby is reading a book and seems to be very reserved.

This directly ties into how adjectives change meaning depending on the copular verb. Ser reservado describes a core personality trait, meaning the person is naturally private, introverted, or shy. If we used estar reservado, it would imply he is only temporarily acting aloof today, or it could refer to a physical object being booked, like a reserved table at a restaurant (a mesa está reservada).

How do we know the gender of the passerby since transeunte ends in -e?

Transeunte is an invariable noun, meaning the word itself doesn't change between masculine and feminine. We know this specific passerby is male because of the masculine demonstrative aquele at the start of the sentence and the masculine adjective agreement in reservado at the end. For a female passerby, the sentence would use aquela transeunte and end with reservada.

Is it mandatory to include ser after parece in this sentence?

No, you can safely drop it and just say parece muito reservado. Both are grammatically correct and very common in European Portuguese. Including the infinitive verb (ser or estar) after parecer is a stylistic choice that slightly emphasizes the state of being, but the meaning remains identical.

Is transeunte a word I should use in everyday casual conversation?