While Sung Jinwoo from the Solo Leveling anime is certainly quite strong, the protagonist is even more powerful in the web novel. Many fans attribute this strength to the System that helped him become the Shadow Monarch. But if he is a Monarch himself, why is he stronger than the other Monarchs?
The truth behind Sung Jinwoo's strength lies in the history of the Rulers and Monarchs. As revealed in the Solo Leveling: Ragnarok web novel, actions once taken by the Absolute Being led Ashborn, a former Ruler, to become part Itarim. Notably so, his successor, Sung Jinwoo, inherited the same fate.
Disclaimer: This article contains spoilers from the Solo Leveling: Ragnarok webnovel.
How is Sung Jinwoo part Itarim in Solo Leveling?

As revealed in the Solo Leveling web novel, the Absolute Being was an Itarim, a race of gods responsible for creating all of existence. He split the Light to create the Rulers and split the Darkness to create the Monarchs. While the Rulers were meant to protect the world and the Monarchs were meant to destroy it, the Absolute Being merely created the two sides for entertainment.
After eons of war between the Rulers and Monarchs, the Rulers asked the Absolute Being for help in defeating the Monarchs once and for all. To their surprise, the Absolute Being refused to respond to them. This led the Rulers to realize that the Absolute Being had only created them for amusement and had no intention of helping them against the Monarchs.

As they began their rebellion, Ashborn, a Ruler loyal to the Absolute Being, tried to stop the other Rulers. Unfortunately, he failed and was defeated by the other Rulers. Little did they know, the Absolute Being suspected the Rulers would rebel against him. Hence, he presented Ashborn with the Primordial Darkness, a power capable of defeating the Rulers and sending any being into nothingness. This development saw Ashborn get reborn as the Shadow Monarch.
As a Ruler, Ashborn already possessed the Primordial Light. Hence, receiving the Primordial Darkness, an opposing power saw both power sources merge. While this development itself should have been impossible, it created the Fragment of Itarim.

As explained in the web novel, following the original Absolute Being's death, possessing the Fragment of Itarim qualified Ashborn to become the new Absolute Being. This meant he was stronger than both Rulers and Monarchs, which was also why they feared him.
So, considering that possessing a fragment of Itarim meant that Ashborn could replace Absolute Being, an Itarim, it could effectively mean that possessing both Primordial Light and Darkness made him a part Itarim. Therefore, with Sung Jinwoo succeeding him as the next Shadow Monarch due to the System, it makes sense for him to also have become part Itarim. This also explains why Sung Jinwoo was stronger than the Rulers and Monarchs.
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