With his great potential and brilliance, he would have been the legendary hero instead of Yosho, but he became obsessed with the Jurai Power, the ultimate power of the Jurai royal bloodline, committing countless destructive acts in his pursuit of more power. Yosho defeated him but for some unknown reason was unable (or possibly unwilling) to kill him. Afterwards, Kagato disappeared into the unknown and his name and likenesses were erased from Jurai's records and cities.
But years later, Kagato would return to pose as Yosho, having greatly increased his power having spent his time in the Darkness of the Universe (presumably Hell). Such power granted him, among other things, eternal youth and a Master Key/Sword of his own which resembled a dark bastard sword, as opposed to Tenchi-ken's tsurugi-like appearance. By posing as Yosho and displaying a fraction of his great power, he was recognized as Jurai's returning King and Emperor. Kagato then declared Ayeka and Sasami wanted for treason, all a part of his plan to eliminate the rest of Jurai's royal bloodline and to get revenge against his former friend Yosho. This forces Tenchi and his extended family to leave Earth and journey to Jurai to clear their names.
After Katsuhito reveals that he is Yosho, Kagato confronts him at the old Jurai Palace. Although Yosho - and later Ryoko - was no match for him, when Kagato saw Tenchi, he noticed his resemblance to Yosho and quickly realized that Tenchi is Yosho's descendant.
Taking Ayeka with him and leaving Yosho and Ryoko severely injured, Kagato told Tenchi that if he made it to the Jurai palace then he would challenge him, using Ayeka as leverage to draw Tenchi into the fight. With the help of his friends, Tenchi is able to ascend to the throne room and fight the false emperor directly. However, Kagato is far too strong and skilled for the fight to go evenly. He decides to instead kill Ayeka and end the fight at his leisure. Seeing what's about to happen, Tenchi is able to tap into his true potential. Similar to the original OVA, the two trade sword blows, and it is again Kagato who falls. He doesn't reproach his killer in death, flashing back to how he lost to Yosho and how similar in the end it was.