Hakoda is Sokka and Katara's father, and the son of Gran Gran. It is assumed that he grew up in the Southern Water Tribe, along with his friend Bato. They were a mischievous pair growing up, and stories of their pranks are still told in the tribe like the Great Blubber Fiasco, and that time when they put a squid on Hakoda's head and convinced Gran Gran he was a Water Spirit. (Bato provided the spooky voice. ^^) When Hakoda was 15, he earned the Mark of the Wise in the ice dodging rite of passage.
Two years before the show began (when Sokka and Katara were age 13 & 12) Hakoda left the Southern Water Tribe along with the rest of its warriors to help the Earth Kingdom fight in the war. Sokka wanted to come too, but Hakoda told him he was needed at home. Hakoda hasn't had contact with his family since, but two & half (or so) years later, Katara and Sokka met up with Bato, who had been left behind in the Earth Kingdom due to serious injuries. According to a rendezvous map he had received from his tribesmen, Hakoda and the other warriors should be near the Eastern Earth Kingdom capital.
Hakoda was most recently seen in episode 216 "Appa's Lost Days". For a brief moment he was shown sailing with a few other Southern Water Tribe boats in a heavy storm.