In a French romance, bearing his name, he is the son of Tristan and Iseult (Isolde of Cornwall), whom a hermit had raised. In his adventures he was helped by a dwarf, Tronc, whom the fairies had given him. This dwarf later became the fairy king, Oberon.
Ysaie married Martha, the daughter of King Irion. Their son was called Marc.
Source
Ysaïe le Triste | Late 14th century or early 15th century