Quinella

Quinella is the actual ruler of the Underworld as the highest minister of the Axiom Church.

Background
She was born 350 years before Kirito enters Underworld for the second time. She was the product of a political marriage between strong families at that time, her father was the lord in charge of the, at that point, Town of Centoria, and was the one who gave the Sacred Tasks. To maintain power in the family he created and bestowed upon her a new Sacred Task: Sacred Arts Research.

History
She discovered new Sacred Arts that the other Underworld Residents hadn't known. By studying the meanings of the words used in the commands, she was able to form the first offensive command, the «Thermal Arrow» and with it, she discovered that killing any other living being, will increase the person's stats, so she sneaked out of home every night to hunt nearby animals and rose her stats at an incredible rate.

Taking advantage of her knowledge, she made people believe that she was gifted by gods, and took the opportunity to start creating the Central Cathedral, and with it, the Axiom Church. She created the Feudal System in Underworld by making the lords into nobles and thus, creating a structure to control the entire Underworld. The restriction imposed by the Axiom Church on lowering the life stats of living beings was created by Quinella to prevent other humans from grinding their authority levels.

As her power grew, so did the tower in the Cathedral. From a certain point of time, she no longer showed herself to those of the world and immersed herself purely in the analysis of the Sacred Arts to transcend the ultimate limit set upon her, «Life». She became anxious because, despite all her power, her life continued to drop and there was nothing she could do about it, until one day, when she was on the brink of death, she managed to find a command that allowed her to see the list of all commands accessible in Underworld, at the bottom of which was one that gave her all the authority from the Cardinal System and so, she became immortal.

Trivia

 * Her story resembles Alice's, since she was also very skilled in Sacred Arts.