|First Appearance||Vol.1 Chapter 7|
Ship turned up on the Second Floor with all the other Regulars. He explored the field around him and then accidentally ran into Anak and Hatsu, who were fighting. He freaked out because of them, but managed to avoid being killed and was going to run away when it was announced that the Regulars had to form teams. He was shocked that he would have to team up with two very powerful Regulars and got scared about it, but realized he had no choice and approached them. They completely ignored him, which actually made him annoyed. He grabbed onto their shoulders as the timer went off, qualifying them as a "team".
He then tried to convince Anak and Hatsu that he was actually not useless, by saying he was trained in the "killer arts". He barely managed to get through Lero-Ro's Shinsoo barrier, with his face bloodied, However, he aced Yu Han Sung's door test, deducing the answer better than even Koon, in only about a minute. He was the only Regular to figure out the whole answer all on his own.
His team went into the Crown Game, to try and ascend to the next level early. Anak sat on the throne while he and Hatsu fought the attackers. Leesoo came up against Ho and Serena, and was able to hold then off for a little while despite being unarmed. However, when they attacked him in conjunction, they gained the upper hand, but Hatsu turned up and defeated both of them easily. He and Hatsu were then blasted away by Phonsekal Laure. Anak stopped attacking and jumped off the throne to make her bet with Baam, resulting in the team's disqualification. Leesoo watched the rest of the Crown Game from his team's cell.
He was then assigned the Scout position and seeded first. He managed to get on Koon's "friend" list during training. In the Hide-and-Seek Test he was put on Team A and used as bait to attract Quant. He managed to get Quant to drop his guard, but the Ranker proved to be tougher than expected and the plan failed. Leesoo then teamed up with a few other Regulars to try and delay Quant and showed courage in trying to fight him, but they were all easily blown away.
After the test, as Koon lamented their failure, Leesoo told him that they had done their best, and that Team B would probably fail as well. He then noticed that Laure had been holding a rude sign over his shoulder and tried to kick him.
Leesoo did not reveal Koon's scheme to the others when Koon manipulated the others into helping Baam in the Submerged Fish Test because he sympathized with Baam. Later on in the test, he stood defensively over Anak when she was attacked by Lo Po Bia Ren after he led Ha Yuri Zahard to the scene. Leesoo was entrusted with the Wing Tree insignia by Yuri to give to Baam. After the test, he showed distress over Baam's death. He felt like he owed a debt to Baam and backed up Hatsu's and Koon's plan in helping Baam's wish: To get Rachel to reach the top, recruiting the others for the cause.
In a spoiler posted by SIU, Koon spoke to him about his sadness over Baam's death. Koon suspects foul play during the test and wants revenge, stating how Leesoo and the others were tools for this. Unlike the others in the Part 1 team, only he and Koon are aware of the fact that Rachel has tricked them. This secret is kept only between him and Koon, as Koon stated that "I don't want to ruin the team that Baam so wished to protect".
It is hinted that when Koon split the team into two, he put Leesoo in the trailblazing team designed to attract attention. It is very possible that Leesoo is the leader of the attention drawing team, since both Princesses Anak and Androssi are on that team and they are unlikely listen to each other's orders, and Leesoo is the smartest.
Koon contacted Leesoo, telling him that Rachel, without a doubt, tricked them.
Koon had just passed onto another Floor and broke the good news to Leesoo. Leesoo said that they tried the 30th Floor test but failed. He also mentioned that they could only try once a year. As Androssi and Anak were arguing over a football game, Koon and Leesoo resolved to meet each other in the factory round.
When the news that Koon had "died" reached Leesoo's team, he watched grimly. When he got the invitation to the factory round, he led the team out the door, telling them that they would find Jyu Viole Grace and crush him.Archimedes, Hatsu slashed at the window in front of him, bemoaning Leesoo's descent into idiocy. That evening, he and his team had a live press conference in which he stated that he wasn't worried about Jyu Viole Grace; however, Koon could see through his bluff.
He, Hatsu and Anak were running from their lodgings to the centre of battle when Androssi appeared, carrying four Regulars with her. They told Leesoo where the FUG had set up base and the three of them left to attack. They put Anak up on the observatory, but this was just to gain time for their other teammates to get into position. Once this was done, Leesoo went with Vespa through the sewers to the first floor. There, they met Yuto and he and Vespa worked together to almost defeat Yuto. However, this plan was foiled by Viole, who appeared in the nick of time. Leesoo sealed Viole's shinsoo by having Verdi steal it and sent Amigochaz and Vespa to capture him. Yuto protected Viole and got poisoned, resulting in Viole using the "Thorn" in order to defeat Amigochaz. He was astounded when both Amigochaz and Vespa were defeated by physical strength alone, wondering how his previous information could have been so inaccurate. He was reluctant to surrender and only did so when Verdi informed him that there was someone at her location. Frustrated, he admitted to himself that he had underestimated Viole due to his thirst for revenge and gave the order to retreat. After giving Viole bullets and honey, he, Amigochaz, Vespa and Laure left for the rendezvous point where the whole team shot each other.
Leesoo was drinking on the poolside when Anak came up to him with Ron Mei, a teammate of the top E-rank Regular, Yama's Mad Dog. They exchanged pleasantries for a bit, but despite the confident act, Leesoo was worried for Amigochaz who had yet to heal and Hatsu whose sword broke. At night he gave Hatsu a cell phone with Emile to seek a new sword with it.
He asked Anak for the disappearance of Hatsu and in the start of individual battles he was chosen as one of 10 Gamblers of the event. During the first round, he bet Miseng but she lost against Prince, Akraptor and Wangnan. Before they started the last round, in which you could get a 3 million point bonus, he was tied with Rodni and, in the battle between Anak and Ran, he bet everything on his teammate. During the last battle he was worried because Anak carelessly fell into Ran's trap. He was disqualified from the bets after the intervention of Parakewl.
During the meeting of gamblers, he was arguing with Ron Mei and acknowledged that Mr. Mask was a fairly intelligent person.
Leesoo was bothered by Anak's words about Viole and Androssi's strange behaviour as they began the Development Tournament.
During the Development Tournament both Vespa and Leesoo were brought up to speed by an injured Hatsu on the creation of the worst weapon and the revelation that Baam was alive. He later met up with a confused Koon who he greeted with annoyance of being kept out of the loop.
He briefly lended his aid against Beta, from his carrier and later forfeited the game to allow Viole to be summoned.
After Reflejo challenged Viole, Leesoo and Vespa went off in search for Novick, one of the hostages. At first their search was fruitless, but after Xia Xia revealed their locations, their luck began to change. After successfully finding Novick they were all flown out of the cavern on Vespa's 'adult' form where they met up with Androssi, Hatsu, Koon, Baam and Rak. After evading a hairy situation the Regulars escaped aboard the Wolhaiksong escape ship.
He was seen getting drunk while the Regulars were celebrating their success in the Workshop Battle.
He was with Prince and Akraptor when Wangnan woke up. He greeted the Regular and told him to take care of Baam as the leader of Team Tangsooyook.
He said his goodbyes when his team were leaving to begin climbing the Tower again.
He called Koon and casually conversed with him about the recent events, while being bossed about by Androssi. As Leesoo was seen with the rest of his team, he told Koon to come up where they'd be waiting for them and then he finally congratulated Koon on becoming "The Three" again.