Sob how do I even write this--- time to not use the shipping goggles
Shishido
Okay, back to basics. They're partners, and the best Doubles partners that Hyoutei has. They stuck together like glue ever since Shishido came back to the Regulars with Choutarou's help, and he's eternally grateful to him. Heck, a part of him still believes that he owes Choutarou a lot for that. However, the partnership is also something a little more than that. After all, not all doubles partners were that close, enough that they spent so much time together that they're practically joined at the hip. (Training after school all the way until midnight is one heck of a dedication too.)
First and foremost, Shishido is and will always be (or as long he still loved tennis like he loved breathing and cheese sandwiches and his dog) eternally grateful for Choutarou's help in a lot of things, but especially for getting him back on the Regulars. Sure, Atobe helped, and in a way Shishido grudgingly is grateful for their captain for that too, but Choutarou had not only trained with him for two weeks and actually went through his somewhat masochistic regimen by slamming balls towards Shishido again and again with his Scud Serve, he was also willing to actually be dropped from the Regulars if it meant Shishido gets to take his spot back. With Hyoutei's rule, that's pretty similar to jumping off a building--- it's reckless, and you can't have a second chance at it if the coach said yes. The fact that someone was willing to do that for him pretty much sealed the deal for him: This is a guy he can trust, a guy who'd watch his back, and someone he'll make sure to do the same for as well. He's someone Shishido can depend on, and while he doesn't approve of the whole self-sacrifice for his sake, it was a pretty good sign that he's a reliable junior and someone he would be considered an idiot for not appreciating.
Aside from that, he's also somewhat of a student to Shishido. Partially because it's in Shishido's nature to take care of others in his own way, and partially because he owes Choutarou enough to believe that at the very least he can help him improve his tennis, Shishido became his mentor in and out of tennis training often. He admires Choutarou's progress as if it was his own, because he knows he's got a lot of potential to be better and they work on that always, and often gives him tips or advices for those things. Choutarou probably is the person Shishido drives the hardest out of Hyoutei because he believes in his potential a lot, not only as his partner but also as a tennis player. He even helps him overcome his weakness in his Scud Serves and was damn proud seeing the hard work paid off. While Choutarou may be the guy with the killer serve, Shishido was still the more observant of the two on the courts and was pretty good at pinpointing the issues that needs improvement.
When it comes to Choutarou's timidness and sometimes kindness, Shishido tends to be hard on him because he doesn't want others to take advantage of it. He's very protective of the junior as he is with all of his teammates, just that it shows more with Choutarou because they're partners and he's always making sure to watch him first and foremost. That doesn't mean he thinks that Choutarou is weak or a pushover per se, because he knows that on the court, as long as Choutarou was confident, he can practically run his enemies down until they can't fight back--- especially when he's serving. Hell, he's even certain that between the two of them, Choutarou would probably win in a fight (not that he'd ever admit to this) because he's got the strength and height Shishido doesn't have.
What Shishido does worry about, both at home and in Johto, is that Choutarou would be too dependent on him. He's quite aware that Choutarou looks up to him as his senpai, and he's seen just how easy it is for Choutarou to choke when something catches him so off-guard he's not sure how to react to it. Heck, when he first arrived in route and got to Goldenrod, Choutarou had caught him completely off-guard with the hug and the declaration of missing him. He cares for his partner and does understand the sentiment, but he's not quite certain how Choutarou would fare if he goes away. He's never really said anything about it before that would cause enough concern for Shishido to approach him on the subject, but if the day comes to it, he'll probably sit him down and berate him about it.
On the plus side--- Shishido also thinks of Choutarou as a pretty cool guy, though not in the conventional sense. He knows Choutarou works hard for his goals, knows that he's always open to learning, and maybe Choutarou being so willing to listen to him also strokes his ego from time to time, but more importantly he knows that he's not the type to give up. If there's any kind of person Shishido can't stand, it's someone who gives up when things get hard. And Choutarou is trustworthy. He's someone he can goof around with, tell things to (even though he doesn't do that), and just someone he knows he can share little moments he'd consider as completely lame had it been with anyone else and Choutarou wouldn't take it against him.
Shishido doesn't always understand him, however. There are times when what Choutarou hints at baffles him and there are times when he would get really annoyed at him when he's dancing on subjects. He's quick to jump to conclusions when Choutarou doesn't tell him things, sometimes thinking it's something problematic (like Choutarou and Rise being awkward because of being in a non-existent past relationship) though he rarely pushes unless it actually affects his performance enough to irk Shishido.
Overall, Shishido adores Choutarou. He's the person he trusts the most in Hyoutei, someone who's got his back and vice versa, someone with a limitless potential for growth (literally and figuratively), talented, attentive and someone who's got the Hyoutei spirit through and through. While they don't always understand each other and sometimes don't see things eye to eye, Choutarou gives him the challenge he needs and in turn he gives him the training that he deserves in order to improve, like a good mentor-student relationship.