Work Text:
Killua loves it when his heart beats fast.
It was what kept him alive, literally. Whenever he had an exceptionally powerful opponent to assassinate, his instinct kicked in, and he felt energy coursing throughout his entire body. His heart thundered, his arms, lungs, and legs screamed and demanded him to move, and the extra burst of strength always gave Killua the lightning-fast reflexes he used to kill.
But what confuses him is that it is no longer life-or-death battles that take his breath away.
The first time Killua’s heartbeat sped up outside of battle was when he and Gon stayed at Whale Island.
It was just before dawn when Killua first blinked his eyes open. Gon’s room was still pretty dark, and only a few slivers of sunlight had managed to slip through the window shades. From his position on the floor, Killua kicked off the covers of the futon and stretched his arms and legs, extending his limbs as gracefully as a cat.
Gon’s deep breathing indicated that he was still asleep. Using both elbows, Killua propped himself up to catch a glimpse.
Unsurprisingly, Gon had kicked off all of his covers again. His body was loosely in the fetal position and facing towards the end of the bed, probably a subconscious effort to avoid the light from the window. A few rays of sunlight fell behind Gon, giving him a halo-like glow. Dark eyelashes fluttered with every breath Gon took. His chest rose and fell slowly, the movement mesmerizing to watch.
The sight of seeing Gon--endlessly energetic and loud Gon--being so reverent in slumber was amusing. Killua couldn’t help watching him because it was fascinatingly ironic; for a kid who climbed trees taller than the skyscrapers in Yorknew City and play-wrestled with animals bigger than Mike, Gon almost looked fragile as he slept on. He looked like what normal twelve year-olds looked like in slumber--peaceful, endearing, angelic.
Badump. Badump. Badump.
A wave of terror washed over Killua when he realized his heart was beating erratically. He instantly flung open the window and leapt out to run three full laps around Whale Island to scan for enemies. When Killua returned to a very confused Gon without finding any threats, he was puzzled; surely, there was something his body was trying to warn him about?
Maybe it was puberty? Killua had been told about the body’s emotions going wild during adolescent development.
The first time his heartbeat went a mile a minute outside of battle caught Killua off-guard, but what alarmed even more was the fact that it kept happening.
For instance, it happened again that time Gon was absolutely entranced by how the light of the sunset caught Killua’s silver hair and kept running his hands through his hair. And it always happened whenever Gon grinned from ear to ear and laughed so loudly Killua’s ears rang.
Even when Gon grabbed his hand not to charge into battle, but to show Killua a new bug species he’s never seen before, exhilaration flooded his lungs, and his heart raced so fast he thought his chest would burst.
But after a while, it stopped catching him off-guard. Killua didn’t mind; although there was nothing like the thrill of a good old-fashioned fight, there wasn’t anything compared to this form of adrenaline.
It was different, of course. In combat, lightning coursed through his veins and chills ran through his body from his toes to his fingertips and every single nerve was on edge. Fear and excitement threatened to consume him and swallow him whole.
But when Killua was with Gon--Gon was always there whenever his heart would start racing out of nowhere--there was no fear. There was no despair, no enemy, no violence. Instead, there was warmth. There was sunshine and smiling and Gon’s laughter and light.
Like now, when Killua was just lying in the grass and Gon gently traced the lines of his palm, it felt like he was basking--almost being smothered--in warmth. For once, both of them were quiet, but Killua’s heartbeat was thunderous, the Badump Badump Badump of his heart almost deafening.
Gon suddenly paused in his tracing and stared at him, amber eyes meeting sapphire.
“What?” Killua said, raising an eyebrow.
Gon didn’t respond, and his eyes kept searching Killua’s expression, as if he wanted Killua to answer something he hadn’t yet asked. When Killua was about to repeat his question, Gon suddenly pounced on him.
“Hey--!” Killua cried out. He immediately scrambled to throw off Gon and start a playful wrestling match, but Gon pinned his shoulder to the ground with an adamant hand and grabbed Killua’s fumbling hand with the other. Without a word, Gon pressed his ear to Killua’s chest, directly where his heart was.
“What are you--?!” Killua felt the heat in his cheeks rising. A rosy blush dusted his cheeks as he kept trying to push Gon off, but Gon’s iron grip was so damn irritating.
Badump. Badump. Badump.
Oh no. Oh God. Gon was going to overhear his heart pounding against his chest. He was going to hear and understand and realize that--
“Wow, Killua!” Gon exclaimed. “It’s amazing!”
Killua finally managed to shove off Gon and scrambled to his feet. His breath was coming in gasps, and he turned his bright red face away. The close contact made him feel like his heart would overload.
“What the hell are you--?!”
“Your heart is beating just as fast as mine, Killua!”
Killua froze and looked back at Gon, who was sunshine and smiles. Again. Like always.
“W-what?” Killua spluttered.
“I wanted to see if your heart was beating just as fast as mine, and it was! That’s so cool!” Gon gently took Killua’s wrist and gingerly placed his hand on top of his chest.
Badump. Badump. Badump.
Killua loves it when his heart beats fast, but his favorite moments are when his heartbeat is in sync with Gon’s.