His introversion is the opposite of extraversion (or extroversion).
The Big Five Personality Traits are: openness, conscientiousness, extraversion, agreeableness and neuroticism.
Extraversion is characterized by sociability, talkativeness, assertiveness. People who are extroverts are outgoing, make friends easily and don't overthink. They thrive off interaction with other people.
Introverts ( people like Kerem), tend to be reserved and they like to be by themselves. They don't trust in other people easily, don't have many friends and don't go out a lot. Their value their solitude a lot.