Which ironborn brother of Balon Greyjoy was omitted from the TV series?

The ironborn brother of Balon Greyjoy omitted from the Game of Thrones television series was Victarion Greyjoy.

Victarion is a formidable naval commander and the Lord Captain of the Iron Fleet in the novels. He is one of Balon’s younger brothers and commands the invasion of the North during the War of the Five Kings. After Balon’s death, Victarion competes at the kingsmoot but loses to his brother Euron.

Euron then sends Victarion to Meereen with a marriage proposal for Daenerys. Victarion secretly intends to marry Daenerys himself and use her dragons to strengthen his own claim. During the voyage, he is badly injured, rescued by the priest Moqorro, and eventually brings the Iron Fleet toward Meereen.

The show combines or removes much of the Greyjoy material, retaining Theon, Euron, and a version of Aeron while omitting Victarion. Euron Greyjoy is therefore the key mix-up: he is Balon’s brother and does appear in the series, but Victarion is the absent ironborn commander.