A colloquial term for someone gloomy or morose; a dark-natured, introverted, or moody person.
彼は根暗だから、初対面だと話しかけにくい。
He's kind of gloomy, so it's hard to start a conversation when you first meet him.
自分で『私は根暗だ』と冗談を言う友達がいる。
I have a friend who jokes about being a gloomy person.
根暗な性格のせいで、彼女はあまり友達が多くない。
Because of her gloomy personality, she doesn't have many friends.