Born with the name Yosho Masaki Jurai, Katsuhito Masaki is in reality the crown prince of an alien empire whose members are almost immortal and probably the single most powerful force in the universe. Yosho fled his homeworld, mainly to avoid being crowned Emperor. This may have been because of his mixed-blood heritage or because he would be forced to marry his half-sister Ayeka or because he simply did not want the responsibility. What is known is that when the space pirate Ryoko attacked Jurai, he used that as an excuse to chase after her and remove himself from Juraian affairs.
Yosho chased Ryoko to Earth, the homeworld of his mother, and defeated her in a tremendous battle. During the battle, his treeship was severely damaged and he was forced to land on Earth. When his Juraian tree rooted itself into the soil of Earth he became irrevocably bound to the planet. Without the sustaining waters of the Juraian homeworld, his tree should have gradually lost power and he should have aged like any other mortal, but didn't due to the influence of the three strange gems he had taken from Ryoko.
Yosho took the defeated space pirate and sealed her within a cave and built a shrine nearby to prevent the local populace from interacting with her. He then proceeded to live out his life as a simple monk, having limited contact with his homeworld. He did, however, take a lover and had several children. Many of these settled on Earth and raised families of their own, some left for Jurai or parts unknown. After seven hundred years he apparently decided that Ryoko had been punished enough for her crimes and allowed his grandson Tenchi to free the space pirate, the first in a series of events that would lead to his eventual reunification with his sister.
Despite what seems to be the case at first glance, Katsuhito has not aged a day in the last seven hundred years. His aged appearance is an illusion he fosters (and in many cases prefers) apparently for the sole purpose of concealing his true nature from the younger members of his family (such as Tenchi) or from his sister. Why he feels the need to do this is a mystery. Given his talent for and delight in teasing, taunting and otherwise annoying the people he lives with and his distaste for responsibility, it may simply be that he prefers to be seen as a silly old man then as a powerful young one. Katsuhito plays the part of the annoying old mentor to the hilt. How much of this is an act is unclear, as he rarely gives a straight answer or a coherent reason for his actions. At least, not ones that don't later turn out to be careful half-truths or outright lies.
One could easily underestimate Katsuhito, but that would be foolish. He is as young as he ever was, and has not lagged in skill since seven hundred years ago. He was considered the best swordsmen on Jurai when he left and has access to the Juraian energy of his treeship as he always did. With access to the Master Key he can produce an energy blade, shields and other feats which he can seamlessly combine with his own superior acrobatic and martial skills. Katsuhito is obviously a top tier combatant, if only for his ability to defeat a fully empowered Ryoko.