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Zenitsu realizes it first; they're back in the butterfly mansion and Rengoku is there, he's so much better, now standing on his two feet (chest completely wrapped in badges but otherwise exposed and of course broad and muscled because life is not fair and it seems Pillars are perfect) and an eye patch covering the left side of his face.
The three boys have been wanting to see him for a while since the whole train thing happened, but Rengoku wasn't allowed to have any visitors until now.
Tanjirou was getting desperate.
"Rengoku-san!"
Zenitsu watches the flame hashira react to Tanjirou's voice, he sees the moment he turns around, apparently forgetting he was talking with Kocho just seconds before.
"Tanjirou, my boy!"
It's not the loud happiness in his voice or the glimmer in his fiery eye or the fact that his expression softens as soon as he looks at Tanjirou, it's not even the fact that the hashira seems to be blind to anyone else around but Tanjirou. No, it's the way the sound of his beating heart changes when Tanjirou approaches.
It's like Rengoku's heart is releasing a melody, one that Zenitsu recognizes pretty well because he's encountered a couple of people in love on his journey.
Although this is honestly difficult to wrap his head around; a hashira, one of the most skilled swordsmen alive, is in love with Zenitsu's best friend.
Right. He's sure that won't be a problem at all.
The flame hashira takes Tanjirou in his arms and ignores all kind of protests the other is making in order to hold him close to his chest.
"You're hurt, Rengoku-san!"
"I'm perfectly fine, my boy," the Pillar insists, sitting on the ground with him in his arms, he puts him on his lap and nuzzles his cheek gently until it makes Tanjirou giggle.
Rengoku's heart sounds like it's on fire. The melody it's making becomes incredibly loud but beautiful somehow, although it's making Zenitsu a little bit uncomfortable.
His love is really strong and intense it's consuming him completely and Zenitsu groans when he realizes that Tanjirou's heart is beating obliviously happy inside his chest.
How? How he doesn't notice it? Rengoku is not being even subtle at the moment, staring at Tanjirou like the boy is something to be worshipped. Even Kocho is rolling her eyes at the flame hashira.
Tanjirou is supposed to have an exceptional nose! How–
"Did you eat sweets, Rengoku-san?" Tanjirou asks, nose twitching adorably.
Kocho covers her mouth to stop herself from chuckling, even the flame hashira looks amused at the question.
"No," he mumbles, right eye glimmering with joy.
"Oh," Tanjirou looks confused for a moment. "I thought... You smell really sweet right now."
"Must be something else," Rengoku says softly, almost with hope, waiting for Tanjirou to come to the conclusion himself.
But Tanjirou doesn't, he's too busy worrying over the Pillar to think about anything else.
However, Rengoku doesn't look disappointed, his grin never fades and his heart beats even more determined now. Zenitsu knows, even though he would have preferred not to, that the hashira is going to do anything in his power to make the boy in his arms fall for him.
They finally get to eat something; reluctantly, Rengoku lets Tanjirou go but kneels next to him at the table. The Pillar then seems to remember about Inosuke and Zenitsu. The boy with the board head is too focused on the food to care about anything else, but Zenitsu does roll his eyes when no one is looking.
"Kocho-san, do you know if Tomioka-san is coming soon?" Tanjirou asks with a hopeful smile on his face. He's really fond of the water hashira because of everything he's done for him and Nezuko so it's not really surprising to hear him asking about him.
"I'm afraid not, Tanjirou dear," the insect Pillar says, looking amusedly over the boy's shoulder.
Only then Zenitsu notices the change in Rengoku's heartbeat. It's intense and almost aggressive, like it's growling...
Like he's... jealous.
"Oh," Tanjirou whispers, not even hiding his disappointment. "I wanted to see him."
Zenitsu shivers; he had no idea that the flame hashira was that possessive.
"You'll be coming with me today, right, my boy?" Rengoku asks suddenly, he's clearly trying to distract Tanjirou. "I need to start training you as soon as possible."
To start wooing him as soon as possible, Zenitsu thinks, rolling his eyes and shaking his head.
"We have another mission first, Rengoku-san," he says. "But we'll head to your residence as soon as we're–"
"I'm coming with you," the flame hashira cuts him off suddenly.
"But you're still hurt!" Tanjirou protests, concern written all over his face. He touches the Pillar's chest without even thinking and Zenitsu would like to tell him to stop because Rengoku's heart changes its pace and it's like he's desperate to kiss Tanjirou until he devours him.
Zenitsu doesn't want to watch that.
"I'm fine," Rengoku assures him, taking Tanjirou's hand a pressing his lips against the back of it. "I want to be with you."
When Tanjirou blushes to the tip of his ears, Zenitsu is sure the journey is going to be an absolute nightmare.