I'd say this one takes place around when they're 8-9.
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Furry Friend
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Early on Suzume had discovered something that she had in common with Hibari Kyoya – and it was the oddest thing, if you asked her (but then again, wasn't he already a bit odd? Wasn't she?). What Suzume discovered was Kyoya's love for small, cute things. Namely small animals, and Suzume too found herself softening in the presence of them.
Though where Suzume smiled and maybe, perhaps, cooed a bit on occasion – Kyoya just relaxed, allowed himself a pet or two, and went on his way.
This discovery was made when Suzume found herself in the possession of a small, very fluffy kitten with electric green eyes and black fur. She'd seen it waddle out of a cardboard box and her heart had melted. So Suzume scooped her up, ever so eloquently dubbed her Kaminari ('Nari for short), and trudged on home with her where she refused to take no for an answer. And much later, Kyoya showed up.
He wanted what he usually wanted – a spar – and wasn't above lunging at her for it (not that she would have refused if he just asked). It was when Kaminari waddled out, fluffy tail in the air, stopping midway between Suzume and Kyoya to bat at a blade of grass that Kyoya froze, relaxed, and turned away.
And as he wandered off, Suzume couldn't help but smile secretly into 'Nari's fluffy head, the feline and girl's eyes glowing bright in the fading daylight.
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Cute little kitty 'Nari.