Korra's uncle and the chief of the Northern Water Tribe in Book Two: Spirits is Unalaq.
Unalaq is the younger brother of Tonraq, Korra's father, making him her paternal uncle. He is a powerful waterbender and a deeply traditional, spiritually focused leader. At the start of Book Two, he urges the Southern Water Tribe to restore its connection with the Spirit World and becomes Korra's spiritual mentor.
His public role as a religious traditionalist hides a much darker plan. Unalaq arranged Tonraq's earlier banishment, seeks to free Vaatu, and ultimately fuses with the spirit of darkness to become the Dark Avatar. Korra defeats him in the Book Two finale, “Light in the Dark.”
The main mix-up is Tonraq: he is Korra's father and initially the chief of the Southern Water Tribe, not her uncle. After Unalaq's death, the Southern Water Tribe becomes independent, and Tonraq is formally appointed its first chief. Unalaq's twin children, Eska and Desna, later succeed him as Northern chiefs.