- "Balmung of the Azure Sky, Orca of the Azure Sea, I will not lose to you!"
- — Sieg —
Sieg's appearance is that of a European knight, similar to that of his idol Balmung. A powerful Blademaster, he wears a full suit of plate armor, the visor raised on his helmet to reveal his handsome face and curly blonde hair. Wave symbols cover the majority of his armor.
Sieg is dedicated to surpassing the Descendants of Fianna, Balmung in particular. Due to his obsession with this he has become a fairly powerful character, clearing tough events such as the Zeit Statue only a few seconds behind his idols. Online he has a fairly wide circle of friends, which includes Yuki-chin, Kyo, Alph, and the character of his real life friend Masaya Makino.
Tomonari himself is rather immature and had been making fun of Mai, which he referred to as his "childish way of showing affection." He had only recently asked her out before he fell into a coma. After waking, he remains Mai's boyfriend and has promised to properly show her how to play The World. Like Mai, he lives in Kanazawa. He later creates the character Kuhn.
Tomonari is a member of his school's gaming club along with his best friend Masaya Makino.
Wanting to teach his girlfriend Mai Minase how to play The World, Tomonari created a character for her and invited her to play with him. However on their first adventure together they encountered Aura who was being chased by a strange PC. That PC was actually a vessel for Skeith who appeared and attacked them. Tomonari was defeated and fell into a coma but Mai managed to escape, only falling temporarily unconscious. Though hospitalized Tomonari would play an indirect role for the remainder of Liminality. Using his character data Tokuoka was able to access The World and investigate by creating a copy of his character named Sieg (2). And two of his close friends in the game Yuki Aihara and Kyoko Tohno would become close allies in the fight against CC Corp. In the final episode Mai linked up to a private server created by Helba and watched as Tomonari's character Sieg fought against Corbenik alongside the .hackers. Following their victory Tomonari was freed from the game by Aura and woke up in the real world.
Sieg is briefly mentioned in Fumikazu Hayami's journal as a powerful character that saved Kazu in a high-level dungeon.
Before falling into a coma, Sieg was on the fast track to legendary status, second only to the Descendants of Fianna. Tomonari's reason for choosing the username "Sieg" was that he wanted to be the wielder, not the blade (Balmung is the sword of Siegfried in Ring of the Nibelung). Like Balmung, Sieg's avatar is almost excessively handsome. Although Balmung had surpassed Sieg greatly while he was gone, his determination has not been hindered in the least. Sieg is among the "ghosts" that helped bring down Corbenik's Supreme Defense, and vowed even then not to allow the Descendants of Fianna to beat him. After awakening Sieg continued playing The World and could be found by Kite wandering around the Root Towns.
- See Kuhn
- Sieg's rivalry with Balmung is completely one-sided. Sieg is obsessed, but Balmung doesn't even recognize Sieg's name when it comes up in conversation.
- Sieg and Tomonari Kasumi have two different voice actors in the English version of the series, though their Japanese voice actor is the same.