A member of the Japanese Hunters Association, Goto Ryuji is widely recognised as one of the world's strongest S rank hunters.
Iron is one of Sung Ji-woo's Summons. He was an ex-hunter who got killed during the Ice Elves Arc.
Tusk is another one of Sung Ji-woo's Summon. He used to be a Powerful Goblin Shaman that got killed by Ji-woo. Tusk excels in magic combat and has firepower greater than S rank hunters.
Thomas Andre is one of the strongest Hunters in the world. He is also the leader of the Scavenger Guild, and is a National Level Hunter.
Igris is one of Sung Ji-woo's Summons. He used to be a Red Knight Monster, and was the first boss Ji-woo had to defeat. Later on, Igris reached the rank of a Grand-Marshal.
Beru is another one of Sung Ji-woo's Summons. Beru was the first General Rank Summon, that JI-woo got after clearing Jeju Island Raid.
Bellion is a summon that Ji-woo acquired when he fully unlocked the powers of the Shadow Monarch. Bellion has the rank of a Grand Marshal and is immensely powerful.
Ashborn aka the Shadow Monarch is the original Shadow Sovereign. He bestowed his powers upon Sung Ji-woo giving him his abilities and summons.
The Monarch of Destruction and the King of Dragons, Antares, is one of its mightiest entities. He is a very tough adversary due to his extraordinary strength and resilience.
Without question, the strongest character in Solo Levelling is Sung Jin-woo, the protagonist.