Scenes in Time
The first time he met Sasagawa Kyoko (beneath the classroom's window, where he'd gone to smoke and she'd gone to take the daily trash to the incinerator), Gokudera blurted out his belief that either she or Ryohei had to have been adopted, because no way in hell could two siblings be so different. The fact that she only laughed in response with the same sort of oblivious naivete common to her brother made him rethink his theory.
When he learned about Tsuna's (blatantly obvious) crush on the younger Sasagawa sibling, he made a remark about how goddamn awful it would be to have someone like Ryohei as an in-law. And then he found himself spending the next hour trying to rephrase it in a way that wouldn't turn Tsuna's face beet-red.
Ryohei's 'sumo competition' excuse was ridiculous, implausible and just plain dumb - but when Kyoko came up to him, all smiles, and wished him good luck with his match, Gokudera was just a little bit thankful for the older Sasagawa's idiocy.
Gokudera never, ever felt jealous of the familial bond between Kyoko and Ryohei. But sometimes, when he saw Kyoko fussing over her brother's injuries on the way to school, he thought it might be nice to have someone else dressing his wounds for a change, instead of his usual haphazard method of yanking bandages into place while wincing (and pretending not to).
The time he heard Kyoko cheering on her brother by shouting Ryohei's usual catchphrase in her cheerfully girly voice, he'd practically swallowed his cigarette at how damn cute it sounded.
Any time that Ryohei came up to him to say smoking was an extremely bad habit that would extremely damage his lungs, he extremely told the idiot to fuck off and chainsmoked three cigarettes just to spite him. However, when he and Kyoko were put on cleaning duty together, and she offered him a throat lasange (because cigarettes can hurt your throat sometimes, right?), Gokudera decided to quit smoking then and there. His resolve lasted all of two days, five hours, eighteen minutes and fifty-nine seconds. The lasange remained unopened in his desk-side drawer until he came down with a sore throat about a month later.
Though tutoring Ryohei for the high school entrance exams had primarily been Tsuna's idea, it was ultimately the notion of spending time around Kyoko that got Gokudera to agree. He later wondered if the massive headache from trying to teach the elder Sasagawa was worth it (and inwardly decided that yes, it was).
Gokudera's dislike of Lambo didn't stem entirely from how annoying the brat could be, though that was certainly a key factor. Another was Lambo's inexplicable ability to ruin every attempt at speaking with Kyoko without alerting her overprotective and overly loud brother. Likewise, part of the reason (though he would never, ever admit it) he disliked Yamamoto was due to how easily the baseball idiot could talk with anyone - and how Yamamoto's goofy grin seemed like more of a match for Kyoko's bright smile than Gokudera's usual scowl did.
In high school, waiting within a roofed bus stop as rain pounded against the world outside, Gokudera kissed her. It was a quick, awkward, spur of the moment thing and he made up a dumb excuse (for which he would later berate himself) as to why it didn't actually mean as much as it did. The smile she forced in response to his claim made his chest tighten, and he nearly kissed her again just to make up for it.
When she and Tsuna started dating, Gokudera stayed a little farther away than he used to. Tsuna took it as a relief and a sign that Gokudera was starting to view him more as a friend than a boss - but in truth, Gokudera simply didn't trust himself not to spoil Tsuna's happiness.
The first time they slept together was an accident. The second, third and fourth times, however, weren't.
Whenever he restlessly wandered the hallways after a night together, she somehow always found him and smiled as she wordlessly offered the box of cigarettes he somehow always managed to leave at the bedside.
He asked her once, why she kept coming to see him when she had Tsuna. She brushed hair back from his forehead and replied that she loved them both. In a way, he understood that sort of dual affection completely.
The only one to ever approach him about the affair was, oddly enough, her brother. He told Gokudera in no uncertain terms that if anyone made his sister cry, he would extremely punch them in the face. Gokudera had replied that if that ever happened, he'd have earned it.
Tsuna's death left Gokudera feeling guilty for two reasons: one, he felt that he had failed Tsuna as a guardian, and two, he felt that if he had spent just a little more time with Tsuna and a little less with Tsuna's wife, things might have gone differently.