Grandfather's real name is Yoshou. He is actually the prince of Jurai.
He left to fight the deamon Ryoko (Ryo-ohki series)and his ship crashed on earth.
This all happened before him and Ayeka were married so Yoshou married an earth woman and settled on
earth. Yoshou is Achika's father and Tenchi's grandfather. In the end of the series we find that grandfather is actually still young looking and has been hiding it with his power. His mother talkes him into dropping his disguise.