Fortunately, it wasn't a total dead end.
The Nen system from Hunter x Hunter might not grant the instant, continent-shattering firepower found in shows like Bleach or Naruto, but its baseline power was still formidable.
Take Zushi, for example. Even as a novice training alongside Gon and Killua in the Heavens Arena arc, his sheer physical durability and striking power drastically outclassed any ordinary human, even without actively deploying his aura. If he hadn't run into a generational prodigy like Killua, coasting past the 150th floor of the arena would have been a breeze.
That baseline power was more than enough to survive a standard zombie apocalypse.
The problem was the ticking clock. The outbreak was imminent, and he had no idea if the zombies in reality would be as weak as they were depicted in the anime.
In canon, aside from their lethal infection rate, an individual zombie's combat power was essentially on par with a low-level goblin. However, the entire Highschool of the Dead series only spanned about twelve episodes, covering maybe a week of in-universe time before the anime was abruptly abandoned.
There was no guarantee that the undead wouldn't rapidly mutate over time, like the horrors in Resident Evil, or that the government wouldn't simply glass the entire city with a nuclear strike.
He needed power, and he needed it now. Luckily, Nen had a built-in fast-track method.
Vow and Limitation.
By binding oneself to a set of excessively harsh, self-imposed rules, a user could exponentially multiply their aura. If standard Nen training was addition, Vow and Limitation was multiplication.
It required three core elements: creating the rule, abiding by the rule, and accepting a devastating punishment for breaking it. Furthermore, because Nen was heavily influenced by willpower, the user had to genuinely, psychologically accept the weight of the pact.
The prime example was Kurapika.
Kurapika had trained in Nen for a significantly shorter time than Gon and Killua. Yet, by swearing a lethal, extreme vow, his power skyrocketed to elite hunter levels in a matter of months. He easily slaughtered Uvogin—an Enhancement Type master who could tank anti-tank missiles with his bare skin—and completely neutralized Chrollo.
Kurapika's immense drive was fueled by his burning desire to avenge his slaughtered clan. Thus, he placed a Vow on his Chain Jail, restricting its use exclusively to members of the Phantom Troupe. If he ever used it on anyone else, the Judgment Chain wrapped around his own heart would instantly crush it.
Suyan's own Vow and Limitation was loosely inspired by Kurapika, though far less masochistic.
Under the effects of his Vow, he gained a secondary application for his parchment. The core mechanic shifted from 'trade' to 'contract.'
By binding someone via this new contract, even an ordinary human with zero prior affinity would rapidly awaken their own unique Nen ability. And thanks to the Vow, Suyan would simultaneously gain full access to whatever ability they awakened. If his contracted partners trained and grew stronger, Suyan's power would permanently scale alongside theirs.
As for the specific rules and penalties of his Vow and Limitation?
Let's just say Suyan had ripped the clauses straight from the ultimate, inescapable modern marriage contract from his past life.
Because the conditions were so permanently binding, he couldn't just hand them out to anyone. His targets had to be of the highest caliber. Naturally, iconic anime heroines like the woman standing in front of him were at the absolute top of the list.
What's the point of having a 2D waifu if you don't secure the contract? Thinking about it, Suyan couldn't help but blink playfully at the bewildered Saeko.
While Suyan made it sound incredibly casual, Saeko felt like her brain was about to short-circuit. Conversations about the 'value of existence' and the 'meaning of life' were equivalent to dropping emotional nukes on a girl who had spent her entire life ruthlessly suppressing her true self.
Could it be?
Saeko stared at the White Oak Bokken that had manifested out of thin air. She had watched it happen. He hadn't pulled it from beneath his coat; it had simply materialized into reality.
Combine that with the cryptic things he was saying...
As one of the most popular clubs on campus, the Kendo Club had its fair share of delusional, eighth-grade-syndrome students. She frequently saw underclassmen wildly swinging their bamboo swords, screaming about "the power of friendship" and "unbreakable bonds."
So, Saeko was intimately familiar with chuunibyou delusions.
But Suyan had never once acted like an edgelord. And more importantly, she had just experienced his monstrous, superhuman strength firsthand.
If he wasn't lying to her...
Does a hidden, supernatural underworld actually exist in our society?
Saeko knew perfectly well that her violent urges had no place in the modern, civilized world. It was the sole reason for her crippling self-loathing.
Countless times, she had cursed the era she was born into. If she had been born in the Sengoku period, she might have become a legendary warlord like Minamoto no Yorimitsu. Even if she didn't reach those heights, she would have at least been allowed to unleash her bloodlust on the battlefield without restraint, instead of suffocating herself in the mundane present.
But if there was a hidden world—a realm of blood and shadows operating beneath the surface—then everything changed.
The purple depths of her eyes began to burn with an unprecedented brilliance as she locked gazes with him.
"Saeko, are you willing to entrust your future to me?" Suyan asked, his tone deadly serious.
"I..."
She opened her mouth to accept, but Suyan gently cut her off.
"There's no rush. Take this contract home with you. Give yourself time to truly think about it. Are you absolutely certain you're ready to embrace the reality of this world?"
Seeing her obvious eagerness, Suyan intentionally slowed the pace. It was infinitely better to lay all the cards on the table and let her make a rational decision than to let her sign it in the heat of the moment and regret it later.
Of course, Suyan trusted her implicitly.
It wasn't just because they had spent the last year training together. As a canonical anime heroine, Saeko was fundamentally written to be an idealized, fiercely loyal existence. Among the vast sea of female characters in anime history, Busujima Saeko stood as an undisputed icon for a reason.
If he didn't trust her entirely, he never would have deliberately bound himself to such an inescapable Vow and Limitation just to contract her.
A wise man doesn't fall in love. Unless, of course, it's a 2D waifu.