弟

How it works

は (topic) raises the topic (弟(おとうと) (little brother)) — “as for my little brother…”. The description 元気(げんき) (energetic) is a na-adjective, and here the plain copula だ (is) closes the sentence. That plain だ (is) (instead of polite です (is)) makes it sound casual, like something you’d say to a friend.