Zetsk looks like a prehistoric man because of his height, build, and face.
Zetsk is very close to his brother Gashta, the only person he trusts. He has a violent character.
Greed Island arc
Zetsk first appears with his brother Gashta preparing to take Tsezguerra's Ren test and along with his brother he passes the test as well as 7 other people. After entering the game Zetsk sits and waits for his brother Gashta in front of the Greed Island Entrance; sitting close by to Gon. Soon his brother walks down the stairs on the entrance and thanks him for waiting. Gashta then says they're being watched and Zetsk confirms it saying that they're several people spying on them and that they're not revealing themselves. Gashta however deduces they're no threat to them and since they could sense them from such a distance they should be ignored. The two brothers then head off further into the game.
The Brothers are later mentioned by the Bomb Devils that they're a duo they should keep an eye on and are mentioned again by Goreinu that they're a famous duo who are very strong and never work with others. The Brother are later shown some place in Greed Island listening to Eeta give an announcement that a trivia game will then be held, with the prize for the game being the Ruler's Blessing (G.I card). After Gon wins the trivia game the brothers confront Gon and group. Gashta comments how Gon and group actually managed to get all of the cards in the game, while his brother Zetsk snickers. Zetsk then tells a man that apparently was associated with the brothers that his job is done and can go beat it and then challenges the group to a winner take all match and it doesn't matter if it's 2 vs 4. Gashta then tells the group it's a match they can't refuse, the brothers however are handily defeated, by the boys.
13th Hunter Chairman Election arc
Abilities & Powers
Being a Pro-Hunter and he and his brother, being on the Bomb Devils watch list, can be speculated he's not only a skilled Greed Island Player, but also a skilled Nen user as well. Still, he was effortlessly defeated by Team Gon.