The Light Emperor is the light half of Emperor Mateus's soul and the final Boss in Final Fantasy II.
Final Fantasy II
In the Dawn of Souls and 20th Anniversary versions, something unforeseen happened when the Emperor was killed: His soul split into 2 entities. While one went to Hell and became the lord of Pandemonium, the other went to Heaven and became the lord of Arubboth. With his new powers, he summoned the fallen souls of Minwu, Josef, Scott, and Ricard to him as representatives of those who fell in the wake of the Empire's conquests.
After they traveled to his throne within the walls of Arubboth, he asked for their forgiveness and offered them eternal life. They are reluctant, and consider his offer. However, the souls of all the people they care about appear before them, and tell them to look past his charade.
With their help, they realize they are being tricked, that this Emperor is no different from the other, and attack the Emperor's light side while Firion's group battle his dark counterpart. As he dies in conjunction with his other half at the hands of their respective parties, the Emperor's light half tells them it is futile that the bloody struggles of violence and war with continue as long as humans exist. Minwu comments at the very end that if anyone can change man's violent legacy, it would be Firion and his friends.
The Light Emperor is quite deceptive in nature; he is also monstrous. he claims to regret the crimes he committed in life, but in reality he is just as evil as his dark half. He went to heaven is very likely by deception and force, not because his genuine atonement which he never had.
In Duodecim, the prequel to Dissidia Emperor Mateus can get a downloadable costume which turns him into the light emperor, this may imply that after the Light Emperor's defeat he fell into hell and he and his dark half merged back into one being.