Learn Russian: Эта девочка — такая зануда, а её брат — настоящий непоседа. - This girl is such a bore, and her brother is a real fidget.

These are called common gender nouns (общий род). They usually end in -а or -я like feminine nouns, but they adopt the grammatical gender of the person they describe. Here, зануда refers to a girl, so it takes the feminine такая. Непоседа refers to a boy (брат), so it takes the masculine настоящий.

Could I say такой зануда if I were talking about a boy?

Yes, exactly! If you were describing a male, you would say Этот мальчик — такой зануда. The noun зануда itself doesn't change its spelling, but any adjectives or pronouns modifying it must switch to masculine.

Even though непоседа takes masculine adjectives here, how does it decline for other cases?

It declines exactly like a regular feminine noun ending in -а, regardless of who it refers to. The adjectives modifying it take masculine case endings, but the noun takes feminine declension endings. For example, in the genitive case (meaning "of a real fidget"), you would say настоящего непоседы.

Why are there em-dashes (—) in both halves of the sentence?