The Crystal Saint (水晶聖闘士,Kurisutaru Seinto?) is Cygnus Hyoga's and Kraken Isaac's master in the anime adaptation, as opposed to the manga, in which it is Aquarius Camus master. In the anime, Camus was Crystal Saint's master.
He was considered a threat by Pope Ares, who forced him into his control with his Genrou Maou ken technique. After an emotional battle with Hyoga, Crystal Saint takes his own life to stop Sanctuary from conquering the West Siberia.
In the Hades Saga, that has nothing to do with the original anime, a memory of Hyoga shows that his master was Camus. Since then, in subsequent storylines or adaptations of past storylines, Crystal Saint is largely disregarded in favor of Camus as Hyoga's mentor.