Kef'ha
The nuatoma that Rock served prior to his slavery was the leader of one of the Horneater Peaks. He was related to Rock.
Rock's nuatoma left the Peaks together with members of his family to gain Shardplate and Shardblade for his clan. Believing that it would be easier to win against a Shardbearer with no Shardblade, the nuatoma challenged Highprince Torol Sadeas to a duel, with Sadeas' Shardplate on the line. The nuatoma was killed in the duel.[1]
It is unclear what exactly was offered as stake by the nuatoma, but Rock and the other members of his nuatoma's household became Sadeas' servants (and effectively slaves) after the death of the nuatoma.[1]
Some members of the nuatoma's family were most likely left behind on the Peaks and did not follow the nuatoma to Alethkar, as Kaladin mentions that Rock had sent for his wife in the Horneater homeland after he gained freedom from slavery[2].