Allen is the first person to find out. And it was an accident, sort of. As much as an accident as months of planning to find Kanda's weakness will allow.
"Kanda! Wait a sec—"
And that was how it had all started—Lenalee running after an eighteen year old boy (admittedly one with decidedly homicidal tendencies) in her thigh highs and miniskirt, (everyone who worked at headquarters was very fond of that skirt—Lavi routinely made it his mission in life to shear off a couple more inches,) when it happened.
Lenalee started playing with Kanda's hair.
Allen opened his mouth to start screaming lamentations to the heavens—when he realized that Lenalee was still alive. And intact. And humming—Beethoven's fifth, to be exact.
This was very difficult for Allen to comprehend. He banged his head against the wall a few times—Headquarters was in a state of disarray anyways, and he didn't think that Komui would notice a few more dents.
It failed to return the scene before him to any state of normalcy.
When Lenalee had finished smoothing out Kanda's ponytail, she gave him a shy smile and—and—
GOOD LORD, WAS KANDA SMILING?!
Allen kind of wanted to cry.
Instead, he scuttled away to report his findings.
"HE WHAT?" Lavi spewed. Allen offered him a handkerchief, which Lavi accepted graciously.
"HE WHAT?!" he repeated a second later, this time with a slice of hysteria. Allen ran in frantic little circles.
"I don't know! It just—it was weird!"
"You didn't eat the eggs, did you?" Lavi asked seriously, "Because. Because the eggs, man, you can't trust those things. I don't think they come from chickens. I mean—y'know akuma?"
Allen had to take a minute to visit the restroom and purge that from his insides. When he returned, Lavi was sitting cross legged on the floor with the air of a man determined to utterly crush his opponent.
"Well," he said calmly, "This is cool. We're fine. Yuu is absolutely hopeless with girls so—so—"
"You said last month that he was gay!" Allen accused furiously, "I've been avoiding all the dark corners because of that, you jerk! Clearly, you information is inaccurate!"
"Well, excuse me, but the last time we were near a brothel, do you know how much interest he expressed? None! Zero! Nothing, I tell you!" Lavi broke down weeping, "Oh, poor Lenalee…what a monster she has been stuck with…"
Allen cried a little bit over that as well.
"But—hey," he brightened suddenly, "Maybe he'll die out on a mission?"
Lavi scoffed, "Yu-chan is immune to death. His hair is too cool."
When it became clear that Kanda wasn't planning on stopping with Mission: Covert Glances anytime soon, Allen's subconscious took things to the extremes.
And it. It hadn't been his fault, really, it had just been so fast—one minute there had been a blur of black and blue lunging towards him, he had ducked instinctively, and the next, a Kanda-shaped hole in the stained glass window, an enraged wail, and then an ominous thump.
Complete silence swept through the research facility.
"D-Dude," Johnny's eyes were huge behind his spectacles, "You killed Kanda."
"I did not!" Allen protested instantly, his face flushed and arms flailing, "It was—suicide! I. This was totally not my fault!"
Reever's head poked out from the computer room, slanted eyebrows nearly touching his hairline, "You killed Kanda?!"
"No!" Allen wailed miserably.
"Oh God," Reever dropped the enormous stack of paperwork in horror, his eyes welling up in tears, "You killed Kanda!"
Allen actually threw himself flat on the floor, "Please don't kill me I had no idea you felt like that but have I mentioned that he's actually not gay?"
Reever let out a muffled sob, and seized Allen by the neck of his collar—hauling the slight boy upright until they were almost nose-to-nose. "Do you have any idea how much money he owed me?"
Johnny made a dismissive noise and turned back to his work—and The Bookman sent the pair of them a truly black look.
"Reever, shut up, he owed us all money."
"…what?" Allen mumbled, more than a little dazed. Reever shook him roughly, looking more than a little crazy.
"A LOT OF MONEY."
General Tiedoll sighed, and lifted his head from his book long enough to say, almost fondly, "Kanda wasn't a very good poker player."
Allen had a feeling that this all led back to him, somehow. He squirmed.
The library doors flew open with a crash as Marie sprinted in, "Kanda's dead?!"
Allen squeaked. Marie cursed in at least eight languages Allen was unfamiliar with, and two that he was. "I was making rent off that guy!"
"A LOT OF MONEY," Reever insisted, and shook Allen again.
"Hey," Johnny sniffled, "Life goes on, r-right guys? We can probably sell his…flower thing…"
"Kanda," a light, girlish voice called, "Are you okay?"
Everyone turned around—Lenalee was poking her head through the hole in the glass and staring down curiously.
Allen was pretty sure he heard a faint grunt.
The entire science department surged towards the window, (and the Bookman had very sharp elbows, Allen discovered grumpily,) clustering around Lenalee.
"He's not dead!" Johnny called unnecessarily. Allen was pretty sure he saw a couple of finders break down in tears.
"Shit," Kanda coughed faintly from down below, and sent Allen the evilest look he could muster, while being covered in horse poop.
Yes, Kanda's fall had been broken by the manure pile.
Allen tried to look fierce. He even managed to hold Kanda's DEATHKILLDIE!glare for about three seconds. And then he hid behind Lenalee.
"You tried to kill Yu?" Lavi repeated incredulously. Allen adjusted the icepack and scowled.
"I wish," he muttered sullenly, snowy hair a little crimson. Lavi looked ponderous at this.
Lavi wrung his hands, "But he fell nearly fifty feet. That should kill even him!"
"Not if there's a manure pile in the way." Allen pointed out.
Lavi's eyes widened in fear. His knees were shaking so badly he had to sit back on his bed for support. "…You pushed him into a pile of poop?"
Allen twitched, "It's fertilizer! And I didn't push him!"
"You pushed him into a pile of poop in front of Lenalee!?"
Allen digested this. His head wound began bleeding with renewed life. "Oh God," he squeaked, "Oh God."
The two of them looked at one another somberly. Lavi stood and began rummaging through his desk, "I'll loan you some paper. You should probably get to work on writing that will."
"Tch," Kanda said from a dark corner just behind them.
Allen screamed so loudly the light bulb cracked.
"TERMS OF SURRENDER!" Allen yelled from the closet. He waved around one of Lavi's shirts frantically.
Kanda stopped slashing at the light fixture to glare at the offending garment. Lavi, who was clinging to said light fixture, let out a tiny sigh of relief.
"No survivors," Kanda whispered to himself, "All shall be obliterated—"
"But Yu!" Lavi pleaded, "Think of all the memories we share!"
Kanda's eye took on a demonic glint, "I'll cut you." he threatened, and took a couple stabs at Lavi's bottom. The redhead recoiled with a shriek.
"Kanda!" Allen wailed from the closet, "We'll give you whatever you want!"
"You make me sick," Kanda snarled, and threw Lavi's bed at the closet. Lavi yelled in protest.
"Do you have any idea how much that cost me!?" he took a wild kick at Kanda's head, and actually connected. Kanda gave a little grunt, and managed to scratch Lavi's leg. From the way he screamed, you'd have thought it'd been cut off.
"OH MY GOD SON OF A MUFFIN, MY LEG," Lavi inspected it worriedly, nearly hyperventilating. A small trickle of blood oozed up from the cut, just enough to make a drop. Lavi fell from the ceiling and pointed at Kanda accusingly.
"I'm bleeding," he said with relish. Kanda frowned.
"Shut up, no you aren't!"
"Cold blooded murderer!" Lavi wailed. Kanda seethed.
"I said shut up!"
"OH NO," Lavi gasped, "I'm going to tell the whole world about you and your abusive ways! This relationship is over!"
Kanda threw his sword down to the ground in his aggravation, "What relationship!?"
Lavi sucked in such a harsh wail of air, Allen, (who was at this point crawling from the wreckage of Lavi's closet,) thought he might have heard a lung pop. "WHAT RELATIONSHIP!?" Lavi repeated indignantly, "IS THAT WHAT YOU THINK OF ME?!"
He began hitting and biting Kanda out into the hallway—and the swordsman seemed too shell shocked by this sudden turn of events to do anything else than back away, until he was finally out in the hall. Lavi leaned in close, "I thought what he had was special," he hissed, and slammed the door.
After a moment of deep breaths, he dusted his hands an helped Allen to his feet. "Ha! Yu is so easy to manipulate."
There was an indignant yell from the other side of the door. "I'LL KILL YOU!"
"With what?!" Lavi yelled back mockingly, and plucked Kanda's sword from the ground, "You seem to have forgotten something."
There was a moment of quiet while Kanda took inventory. And then came the explosion.
"You have five seconds to live," his muffled hiss came, even while he tried to kick down the door, "Give me back my sword."
Lavi made a great show of opening the window as loudly as possible.
"Swear you won't go near Lenalee, and I will," he taunted.
Allen went back to the closet. He was pretty sure only Lavi could be this stupid.
Kanda got very quiet, suddenly. Too quiet. Sudden death quiet. A killer aura began to leech its way under the door.
"I don't know what you're talking about," he said steely, "The next time you say such a stupid thing, I'll cut out your tongue."
Lavi laughed like a madman. Allen decided to leave Reever his shoes.