His opponent, Paul Butler, was in survival mode throughout the fight, very reluctant to throw punches for fear of opening his guard.

Butler knew what he was dealing with: not just any boxer, but one who had earned the nickname “The Monster” for his untamed strength, lightning-fast hands and uncanny operate of angles.

Over the years, the Japanese star has easily climbed through various weight classes, and her triumphs have given Naoya Inoue an almost mythical status.

Butler, a solid contender who held the last belt standing between Inoue and the undisputed bantamweight championship, learned firsthand what makes Inoue so special.

Despite being in the high guard for the entirety of their December match, Inoue still found a way. That he was able to stop Butler wasn’t all that impressive. That Inoue (24-0, 21 KOs) was able to do it despite Butler’s reluctance to fight? Incredibly sporadic. Fighters often criticize their opponents for “running” to explain why they didn’t get a knockout in a dull fight. In Inoue’s case, there’s no need for excuses.

He burst into the American boxing consciousness with three title knockouts in 2018-19 in Japan over top-10 bantamweight contenders, all of which were televised in the early morning hours in the United States.

Now, Inoue, ESPN’s No. 2 boxer, is ready for his toughest challenge yet, another chance to build on his legend. In his first fight at 122 pounds, Inoue will face the top junior featherweight in the world, Philadelphia’s Stephen Fulton, for the WBC and WBO titles Tuesday in Tokyo (4:30 a.m. ET, ESPN+).

Fulton (21-0, 8 KOs) is a talented fighter at any weight, ranked just outside the top 10, and is significantly larger than Inoue (he won his first title at 122 pounds). Inoue won his first title at 108 pounds, and so far his talent has proven too much to be contained by size.

“I’m pushing the limits of my physique, my limits,” Inoue, 30, told ESPN’s “Camp Life.” “I really don’t know what kind of fight it’s going to be. I’m just going to do everything I can to win. If I get the chance, I’ll knock her out, and if I don’t, I’m going to focus on staying in good boxing shape to win.”

How good is “The Monster”? Let’s take a look at what makes Inoue so special ahead of his matchup with Fulton.

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“Inoue has great eyes”

Inoue rarely makes a mistake, despite his offensive nature. In his first three fights at 118 pounds, Inoue has crushed the competition in a flash. First, in May 2018, he scored a stunning first-round TKO of Jamie McDonnell to win the bantamweight title. Five months later, Inoue dominated a bona fide top-10 bantamweight contender, knocking out Juan Carlos Payano in the first round. His best win at bantamweight came in his next fight, a second-round TKO of Emmanuel Rodriguez in the World Boxing Super Series to win another title at 118 pounds.

Throughout all three fights, one attribute stood out: Inoue’s vision. His ability to catch his opponents’ punches while also staying in position to capitalize on opportunities makes him threatening.

“Inoue has great ‘eyes’ and a steely calmness that allows him to see where the openings are and the proper positioning for specific punches,” famed trainer Teddy Atlas told ESPN. “His extraordinary confidence and belief in himself through these attributes, and his sense of timing and foot placement that give him solid balance when throwing punches, make him incredibly effective on the offensive.

“Add to that his good hand coverage habits and overall solid fundamentals, and he’s a real “monster” to deal with.”

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“He is cruel”

Inoue had faced little adversity in his first three fights at 118 pounds. None of them made it to the third round, but in his fourth fight, things changed dramatically.

In the World Boxing Super Series finale in November 2019, Inoue faced future Hall of Fame inductee Nonito Donaire. The brutal fight, named Fight of the Year by ESPN, showed exactly what kind of character Inoue has.

The then 26-year-old suffered a broken nose and a fractured orbital bone in the first three rounds but never wavered. Inoue continued to attack the impressive puncher and broke in round 11 when he dropped Donaire with a body blow.

Donaire seemed deterred, but somehow he heard the final bell as Inoue punished him in the final two rounds despite suffering two contusions that narrow his vision and ability to breathe.

“He was tough,” Donaire said. “I didn’t expect that from him. He can crack, too. … He’s got some speed. He’s got all-around [package]“The Filipino Flash” said on the DAZN Boxing Show ahead of his June 2022 rematch with Inoue. In that second fight, Inoue left no doubt, defeating him with absolute devastation, knocking out Donaire in the second round to claim his third bantamweight title.

In Round 1, he knocked Donaire down with a powerful right hook – Donaire said it was the hardest punch he had ever received – and then Inoue knocked Donaire down again in Round 2 before winning by technical knockout.

“He’s a mean son of a bitch,” Mike Tyson said on his “Hotboxin’” podcast last year. “… He doesn’t look like much. He’s better than Manny Pacquiao. … He’s vicious. … He’s a f**king monster.”

Hall of Fame promoter Lou DiBella, who oversaw HBO’s boxing programming from 1989 to 2000, shared Tyson’s assessment.

“He’s a complete fighter, and that’s what separates him from a lot of other Japanese fighters in history: he’s a badass,” DiBella told ESPN. “He’s got the mentality of a badass killer.”

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“He is the best fighter in Japanese history”

Inoue isn’t yet a star in the States. After all, he’s only fought three times in the States, and the rest of his fights have come in Japan, where he’s an icon. Most of his recent fights have been broadcast on ESPN+, but in the morning on the East Coast and early morning on the West Coast.

That lack of visibility has made it hard for Inoue to break through in the U.S., but there’s no mistaking what he has in mind for Japan.

Fighting to sell-out crowds in his home country and generating huge ticket sales, Inoue routinely draws millions and millions of viewers. And in a country that is boxing-mad, he stands head and shoulders above the rest.

“Japanese boxing has a prosperous, century-long history… he’s the best fighter in Japanese history,” DiBella said. “He has no weaknesses. Fulton is a truly incredible talent, he’s not without his chances here. But ‘The Monster’ doesn’t do anything wrong. You have to fight him perfectly to have a chance. He’s just an extraordinary fighter and a great fighter to watch.”

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“Very good boxing IQ”

Inoue is not only a fearless boxer with knockout power in both hands and exceptional athletic abilities. In addition to these physical attributes, he also has ring intelligence.

Former welterweight contender Dmitry Salita witnessed Inoue’s boxing mind firsthand. Salita promoted Antonio Nieves, Inoue’s opponent in his U.S. debut, and watched Inoue toy with the American in an HBO triple header in September 2017.

“He has a very good boxing IQ,” Salita told ESPN. “… With Inoue, as well [Terence] Crawford, he’s a very good judge of distance and he knows how to change speeds. He knows how to change his power shots. Sometimes he throws punches to loosen up his opponents and make them feel sheltered.

“He’s got a few different power levels. He’s got lightweight punches, medium punches, and he loosens up his opponent, so he thinks he’s a good puncher, but not a massive, massive puncher. He can throw punches and change different speeds throughout the fight and the round. He’s extremely explosive and can really go from 0 to 60 in milliseconds.”

Salita is also impressed with Inoue’s disciplined approach.

“He seems to have been built mentally, physically and spiritually to be an exceptional fighter from a newborn age. When the going gets tough and you get tired, you can operate that education you got from a newborn age.”

In miniature, to quote DiBella, “If you don’t love this kid, you don’t know anything about boxing.”