During his first appearances, Nils only cared for science, he was never into Gunpla Battle for the thrill of it, but rather just to study Plavsky Particles. After his battle with Sei and Reiji, he realized he in fact enjoyed Gunpla.
As a Gundam Builder and Operator, Nils employs advanced knowledge and natural skill (most likely gained from his martial artist abilities) along with the expert craftsmanship and tuning of his Sengoku Astray in order to win his battles by using tactical decisions. While he shows no apparent gratification over his victories or concern for the nature of battles themselves, believing them to be a stepping stone in his main goal of advancing his research on the Particles, fighters like Ricardo Fellini and Meijin Kawaguchi have attempted to point out the emotions that drive the pilots engaging in these fights, and at least Fellini seems to believe Nils could come to enjoy the less practical achievements of being a Gundam Fighter.
Nils is the Gunpla Fighter from and representing America. His Gunpla is the Sengoku Astray Gundam. Nils' father is a famous detective and his mother is an expert in martial arts. Because of the three Ph.D degrees he earned at a young age, he is nicknamed the "Early Genius." He is also nicknamed "Samurai Boy" for his traditional Japanese outfit and mannerisms.
Nils' main goal is to win through the Gunpla World Tournament and obtain information about the secret of the Plavsky Particles by meeting the founders of PPSE. He appears in the America's Qualifier tournament's final round, facing against Greco Logan and his OZ-00MSVa Tallgeese Valkyrie. Nils is able to make full use of Plavsky Particles in his attacks of the Sengoku Astray during his Gunpla Battle with very little effort and annihilates Greco's Tallgeese Valkyrie.
Nils discreetly assisted Ricardo Fellini, Mao Yasaka and Reiji in destroying the Mega sized Zaku in the second eliminatory match, by rescuing Fellini with the use of his sword. He claimed to have done so in order to evaluate Sei Iori's Discharge System Rifle Mode, but Fellini points out that might not have been his only reason.
Nils has also employed underhanded tactics to learn more about Plavsky Particles by using his martial arts skills inherited from his mother and his detective skills inherited from his father to spy on PPSE's Chairman and his PA at their mansion. It led him to confirm Mashita held deep secrets on the Plavsky Particle. While doing this he learned one of the tournament contestants also knew, but did not remember, the secret, but not which contestant.
After defeating his opponent's Gundam Virtue, Nils was confronted by Baker, who wanted him to join PPSE to which he agreed but only if he was able to see the factory where the Plavsky Particle system was created. Baker agreed to the terms but would only allow it if he got into the quarterfinals of the tournament.
Back at his apartment, Nils questioned why PPSE would only allow him a tour of the factory after the quarterfinals and then he remembered the conversation between Mashita and Baker after he spied on them. It led him to assume the boy that knew the secret was either Sei or Reiji and decided to find out what they knew until a mysterious figure blinded him, which triggered his instincts to defend himself. The figure turned out to be Caroline who wanted to see how her "boyfriend" was doing, much to Nils' surprise and consternation.
At nightfall, Nils met Sei and Reiji at the park and asked both of them if they knew anything about the Particles to which they both had no information besides knowing they move Gunpla. Nils asked them to give him any information if they recalled something about the particles and if they did then he would withdraw from the battle, which angered both Sei and Reiji, who believed that his way of fighting to know the Particles dishonored Gunpla battles and negotiations that night ended.
On the day of their battle, Nils's Sengoku Astray deflected a shot from Star Build Strike's beam rifle with its particle blades which forced Sei and Reiji to purge the Build Strike of its equipment and activate the RG system. As the battle intensified between the Astray and the Strike, a pause in the battle (Baker's doing) allowed both sides to repair their models and after checking his Sengoku Astray's damaged body, Nils found himself wanting to pay the two back for hurting his "perfect" Gunpla. With the repairs finished, the battle resumed as Nils attacked ferociously at the duo but at the end lost the battle after the Build Strike avoided the Sengoku Astray's Particle "fa jin" attack and destroyed it with its RG Build Knuckle.
After the match, Nils confronted the duo and wanted a rematch next time they meet. Rather than angered, both Sei and Reiji were amused. Reiji quipped that Nils actually did love Gunpla after all, and Sei went on to apologize for their earlier confrontation. The conversation ended with an agreement to meet Nils next time in battle. It was here that Nils realized that Gunpla meant more than mere fights with toys and a means to an end to him, admitting that he loved Gunpla after all.
Like Mao Yasaka, Nils started to spend the time outside of the contests with Sei and Reiji, trying to get either of them to remember the secret of the Plavsky Particle creation. After Sei admitted his father was involved in the testing of the system, Nils attempted to extract the secret by hypnotizing Sei, which accidentally led the boy to recite all 43 episodes of the original Gundam series. This made the group late to Aila's battle with Ricardo Fellini.
During Reiji's own battle with Aila, Nils was one of the people present at the arena when a strange effect caused a blinding glow to erupt from the battle system (actually from Reiji and Aila's Arista resonating with the field particles), it was Nils who pointed out the increase in the Plavsky Particles when the phenomena occured, which was odd since the Battle System could only disperse the Particles already stored in the machine, and not create more of them.
Later, during the Gunpla Eve festival, Nils didn't enjoy the festival like other fighters did. Instead, he tried to follow PPSE's truck that refills Plavsky Particles. As Caroline made another surprise arrival that scared Nils in order to assist him, the truck got away. Caroline, however, ordered one of her servants to follow it. After tracking the truck, they discovered a hidden passage and realized they're just below the Tournament Arena. The hidden passage led to a huge Plavsky Particle Crystal connected to some tubes that lead directly to the arena, but just as they were going to make their escape, they were located and captured by Baker and other PPSE's workers.
Nils and Caroline were bound together within the chamber, making repeated attempts to escape that ended in failure. During the fight with Meijin Kawaguchi in the Finals, however, the Crystal repeatedly reacted to external systems used to control the Meijin, allowing Nils to start deducing what the effects of it could be. The match progressed and the Crystal continued to resonate until the fight's end, where it reacted one final time to create a massive interpretation of the A Baoa Qu fortress that was large enough to break through the stadium's ceiling. Smaller crystals also formed on the arena. Nils and Caroline took this opportunity to escape and warn their allies, but it was too late.
Deciding to fight, Nils deployed into combat with his repaired and upgraded Sengoku Astray, now sporting a hammer and battle axe. He easily cut and smashed his way through waves of Mocks with his skill and Gunpla's performance, eventually drawing out his Katana's to use Particle Ki-Blasts and swiftly destroy various waves of enemies. Nils also defended Caroline from a Mock that nearly destroyed her Gunpla, but at the cost of one of his Katanas and his shield being destroyed from a shot to the back. Nevertheless, he continued fighting fiercely to allow Sei, Reiji, Aila and China to infiltrate the fortress and destroy it. He watched in shock as it fired a massive Particle Cannon from within and also witnessed as the core of the fortress (the Plavsky Crystal) was obliterated.
Following the fight, Nils stood with his allies as Reiji and Sei managed to fight one more time against Tatsuya Yuuki, but their fight was cut short at the last second as the Plavsky Particles finally ran out and their Gunplas went back to their inanimate state. He also watched in slight disbelief as the two friends bid their farewells to each other.
Much later on, Nils and Caroline were capable of achieving enough data to rediscover the Plavsky Particles, being met with acclaim from the worldwide media.
Seven years later, he is now married to Caroline Yajima(something he seems far more accepting of now that they've both matured), and is the chief engineer at Nielsen Labs and Yajima Trading. Nils no longer partcipates in championship tournaments, as he feels his extensive knowledge on Plavsky Particles gives him an unfair advantage.
While Nils has so far not directly engaged in conversation with Sei. The specialist praises the latter's ability to develop Gunplas after witnessing the effects of the Star Build Strike. He is also aware of Sei's association to China Kousaka, who was able (accidentally) to exploit a flaw in his strategies during a Girls Only Match, granting her victory over the girl Nils had been coaching.
At the Seventh World Tournament First Battle Nils is impressed with the Discharge System Boost Mode, then at the second round he rescue's the Gundam Fenice (and by extent protects the Strike) in order to learn more about the Rifle System. At the 26th Single Match of the Eight Round Eliminatory Matches Nils, along with Mao (correctly) interpret the functions of the RG system, he then watches on with a semblance of seriousness over the end of the match
Nils had at one time assisted in the building of a Gunpla for Caroline (whose father's company was his sponsor in the World Tournament). The Gunpla constructed was the SD Knight Gundam who would face China Kousaka's Beargguy III. With Nils training Caroline and Sei and Reiji training China.
Nils tactics would have allowed Yajima to win the battle had it not been for China's use of cotton on her Gunpla, a material that was normally not included in the crafting of Fighter Grade Gunplas. Inspired by the event to think outside of Gundam lore, Sei later used what he learned here to build the Star Build Strike from scratch.
Despite the loss, Caroline developed a crush on Nils, outright stating him to be her boyfriend. However, Nils initially accepted the role far more reluctantly. After his loss to Sei and Reiji she attempted to comfort him, telling him that her father's company would continue to support him, and that she would as well, after all, she was his girlfriend! It seems that in this moment he let go of his reluctance, realizing that, while she was pompous and overbearing, she was also genuine. Later on, when he was tracking the particle tank trucks she surprised him and called him her fiance. Rather than being irritated with her he was simply shocked that their relationship had moved forward, with no input from him!. After the 7 year time lapse, it was shown that he married Caroline.
Nils assisted the Italian Champion in the second round duel by intercepting the attack on the Fellini's Wing Gundam with his Sword. Fellini then went to thank him and both argued that while the "Early Genius" had assisted in order to learn more about the Discharge System, at the same time the "Samurai Boy" had interfered out of desire to fight strong opponents in their top form rather than letting external circumstances (in the case, an artificial enemy) eliminate worthy opponents.
During the 26th Single Battle Match of the eighth round in the World Tournament Eliminatory Nils expressed confusion over Fellini's insistence on fighting the Star Build Strike in an all out battle when Fellini already had a spot in the finals, not to mention the extensive damage to the Gunpla. Tatsuya (as the Meijin) then pointed out Nils didn't seem to enjoy Gunpla, and told him to watch a fight between real Gunpla fighters.
Notes & Trivia
- Nils was named after real life researcher Nils Nilsson, the first Kumagai Professor of Engineering, most known for his contributions of search, planning, knowledge presentation and robotics, fields which Nils himself had displayed during the series.
- Nils seems to have an affinity for metallic paint jobs. His customized Hyaku Shiki, although it does not have this, was originally a golden mobile suit. His two later creations, the Sengoku Astray and the SD Knight Gundam he built for Caroline, has them.
- Nils had the Sengoku Astray ride a Fuunsaiki Gunpla during the racing round of the Gunpla tournament. This ties into the Musha Gundam theme, and might also be a reference to the mobile suits from Mobile Fighter G Gundam being adapted into Musha Gundams in Superior Defender Gundam Force (most notable of which being Entengo).
- Nils has 3 Doctorates in 3 Ph.D. degrees.