He is a pure, innocent, and gullible teenieping.

He is childish and often gets carried away by other people's stories and is often fooled.

When he realizes he has been tricked, he gets sulky, but soon believes again.

His gullible nature unintentionally leads to mishaps. However, that gullibility is often the key to solving a case.

When he gets sulky, he sometimes sits down under his umbrella.