Koukane is beautiful in all flavors, a prime rare pair material, so let's talk about them!
Kou is easily overwhelmed by Akane, kind of stumbling into being his friend when it clearly wasn't in his plans.
Their first interactions are adorable in a pitiful way because Kou can't keep up with Akane's mood swings, he is so nervous he calls Akane "Sir" completely umprompted, constantly being at lost for words near the older boy.
And while Kou aknowladge Akane can be scary and unhinged:
This nervousness is born from admiration, he gets flustered near Akane when the older boy isn't even trying. Stuttering and startling and awkwardly reaching out for him without daring to approach.
His habit to awkwardly reach out likely involves wanting to help Akane, he just can't force himself to actually close the distance and be of help.
The one time Kou was able to help, delighted to the point of happy tears by the opportunity, Akane killed part of his pride by showing off a power Kou seems to think is much cooler and stronger than his own, judging by the way his big blush turned into sweat.
To further stab his poor heart, when he was honest about how cool Akane powers are, Akane dismissed his own as pathetic. So I don't blame Kou for being so nervous about trying to help his 'cool upperclassman' during the entire arc.
Akane has no nerves when it comes to Kou so he is the one that always approaches Kou and get them closer,declaring Kou his friend, kicking Hanako when it looked like he was mocking Kou's desperate determination to save Nene, and being the only one who touches Kou, Kou never dares to touch Akane.
Surprisingly, Kou's respect seem to extend to Akane's view on Hanako, cause we don't see his reaction to Akane's "you're a murderer, you make me sick" rant, but we do know Kou isn't nearly as unseatled as Nene about it, he isn't even angry, when he brings it up again later with Hanako, he is pretty casual about it. And in the recap he is just confussed.
Maybe Akane reminds him of Teru, and he is used to such a harsh stance, or he is thinking of the young exorcist arc where Hanako's beliefs were the same as Akane's 'I am being nice now but i murdered someone kid. What would ever justify murder? How can a murderer ever be good?' so I think Kou is mostly shocked someone with Akane's belief decided to go through with the alliance anyways. Teru would never.
Back to Kou being extremely nervous!
During the clock keepers arc, after Akane dons his clock keeper uniform, Kou never disagree with Akane directly. Even Kou's habit to back up Nene doesn't get in the way, hyping Nene up but not agreeing with her contrasting views like "hanako isn't bad" or directly refuting what Akane says.
This hesitation/nerves slowly dissapears as they grow closer, nowadays Kou isn't scared of speaking his mind, able to be extremelly rude and not filter his offence.
We aren't show them interacting much in the main manga but we are told that they do talk outside classes, likely alone (?), so Akane's declaration they're friends was not a thrown away line, he genuinely vibe with Kou, he even gave him his phone number.
Kou got more comfortable with Akane, able to express that the boy is a weirdo but he still respects him, seeing him as a wise guy following his lead without question: He is excited to her Akane's romantic advise and listened to Akane's plans even when it comes to his own brother.
Akane likes Kou, he is the one who declared them friends. He admires his determination and want him to succedd in his goals, he wouldn't have stopped his time on what he consider an 'imposible task' to save Nene otherwise. He also really appreciates that Kou doesn't like Aoi. And he is constantly comparing Kou with Teru, baffled by how much they differ.
Akane also grows to likes when Kou is cheerful and full of expectations, either overwhelmed and doing as Kou want or charmed, unable to resist the urge to tease him.
There is a translation where he refers to Kou as Teru's 'cute little brother' and considering how he treats Kou, even if it is a mistranslation i wouldn't be surprised if that's how he think.
Specially with the way he acts as if Kou is too good to be related to Teru, touched by his kindness.
Akane also loves his food, blushing when he eat it. He respect him as a chef.
Akane trying to cheer Kou after his fight with Kou up is really sweet to me because he sucks, they aren't close enough for hugs or shoulder pats but still enough for Akane to take his distress seriously. He is unconfortable and sweeaty, awkwardly reach out, as if he want to help Kou but has no idea how. It remind me of Kou's own awkward reaching out gestures in the clock keepers arc.
Their friendship is beautiful and funny but every flavor of koukane is good, so let's add a puppy crush on Kou. He already finds Akane attractive.
This is attraction by the way, he isn't flustered because Akane is being suggestive. Even if we put asside how pretty Akane was drawn here, Kou is not weak to sexual comments, he is used to Hanako, whose pervy jokes are far more explicit, but he isn't a fan of them, always gets mostly annoyed not flustered.
In the spin off it is revealed a girl Hanako isn't 'his type', but when Hanako possessed Nene's body (who Kou used to have a crush on so he consider her an attractive girl) he becomes imcapacitated. It is a big weakness.
So Akane's smirk is what got Kou flustered. Reading Kou's nerves as half admiration and half the poor boy feeling way too bi to know how to handle Akane would be fun.
I can imagine Akane talking about Aoi, since Kou is one of the few boys who doesn't have a crush on her and Kou feeling awkward cause everytime Akane send him a cute smile thinking of Aoi, Kou short circuit.
Considering we only see Kou call Akane "sir!" after his transformation I headcanon he is more attracted to Akane's keeper form than his usual apparence. If Akane teases Kou while on the keeper uniform Kou may explode.
If we keep this ship one sided, it would never leave the dock, Kou would never go after Akane, but I can see him trying to help Akane get with Aoi when they become close friends, similar to how he had helped Nene with Hanako during the donuts chapter. Boy got the 'he is so out of my league i won't even try' doubts mixed with a sprinkle of i don't like boys' cause internalized homophobia is one fo the ingredient of Kou's 'i want to be tough and reliable! I don't want to be a housewife' complex.
(Just realized Kou think using your bare fists to fight is the peak of toughness and guess what Akane's fighting style is. Guess.)
Akane liking him back is amazing because they both got the acts of service displays of love, something Akane will be touched by and Kou will be comflicted because he is the acts of service guys that's his one thing he can do right, let him do stuffs!! What Kou is weak to is words of affirmation, so he would explode if Akane says he does a good job, is handsome, or kiss well or anything really.
Akane can consistently overwhelm Kou when he is being confident so if he say "Sit down I'll clean today," Kou would try to argue but inevitably cave in and have to awkwardly let someone help him. I am telling you right now, breakfast will become Akane's role.
Kou may be a hard working boy that carries his family on his back but he is NOT a morning person. A single sleep over would be enough for Akane to notice that and let his wondeful boyfriend rest. Depending on how sappy he is feeling, or if it is a important date like birthdays or valentines, he'll give Kou breakfast in bed.
Kou being easily overwhelmed/having low self steem and Akane's habit of being very open about his love, puting his partner first, would result in Kou being super flustered and flattered and uyguygyugu at the start but slowly turn into a problem if unchecked.
Akane is reliable so he can help Kou with chores, studies, or supernatural problems, which Kou would appreciate, and rely on, but it would slowly make Kou feel useless by comparation.
(It would feed into Teru's fears that his siblings are growing out of him too buuut i'll make a post on Teru's amazing potential roles/affect in the ship for later)
Even when Kou does help he has a habit of focusing on what he can't do instead of what he can, feeling like all he does right is cook, and that Akane doesn't trust him, since Teru is the one Akane goes to when it comes to supernatural troubles and help him in the student council. He worries about Kou more than he ever relied on him, at least, when it comes to battle.
Unlike with Mitsuba, I can't see Kou being angry Akane doesn't rely on him, there was never any illusion that Kou can 'grant his wish' or that he is the only one trustworthy in Akane's life. Kou is the one who usually seek him for help from the start, and he respect the people Akane work with more than he respects himself, so he would just be extremaly frustrated with himself that he isn't capable enough to be Akane first option in troubling times.
Thankfully, unlike Aoi, Kou's poker face sucks, and he is an incredibly bad liar, when he bottle up his emotions it easily overflows. Anger, sadness, nervousness...
Just look how he acts with Nene after he learn about her lifespam, how easily his smile breaks after his visit with mitsumom, how reactive he get with Hanako, and so on...
So Akane, who is used to playing hard mode (since he grew up reading Aoi's eternal smile) would notice all of Kou's mood swings right away. If Kou dismisses it, Akane will let him be, but when it keep happening, or he is visibly very troubled, a talk would occur.
I think the first time a misunderstanding would hapen relatively earlier, over something small: Such as Akane thinking Kou is cute when Kou wants to be cool. Kou would be embarrassed at first, he is never complemented on his apparence in the manga in any way, in fact, he is comparaed to a delinquent in apparence, but find it kind of pathetic after a while that the main thing Akane calls him is cute.
Imagine Kou trying out cooler clothes and Akane being somewhat baffled but endeared. Akane won't even question, if Kou want to have a punk era, good for him, but when he finally connect the dots at his anxious behavior he will figure out exactly what kind of compliment Kou craves and overwhelm the poor kid.
Eventually every little piece and small issue would connect and Akane will call him out on his "I need to be useful" complex. Probably result in tears from Kou and regular training sessions.
Kou is incredibly powerful, he is a Minamoto! The only one who caught Mirai mid air, and all the monster he killed at the aquarium was while his weapon was sealed, so Akane would absolutely grow to respect him. Kou would also be a non tradicional fighter, knowing some tradicional exorcists attacks and some judo tecniques.
Akane consider getting homemade food the peak of displays of love (it's a fantasy that is brought up a LOT/ he was extremelly flustered when Aoi gave him a little homebaked sweet)
So despite Kou not thinking much of every bento he makes for Akane, his upperclassman would be delighted. Specially when Kou's meals become more and more tailored to Akane's tastes everytime Kou learns more about his preferences, made with Akane in mind. Add in how Akane naturally loves Kou cooking, even ship lenses, and every meal will be Akane's favorite time of the day.
Instead of leaving Akane to his fate when he is having even day to day troubles Kou would try to help now that he isn't so nervous anymore, and can move instead of reaching out and awkwardly staying in place. Just like when Tiara was troubling him and Kou immediately went to help.
They teamed up for damage control flawlessly. Just saying if they ever live together the chores would be split and all would be nice.
Akane always try to do things alone, and we are constantly said Akane get stuck with jobs no one want to do, helping others but not asking for help. Kou naturally always tries to do things for others, so he would tell him to relax when he is too tired and offer help in small tasks he can, like delivering papers to various clubs, things he would consider pretty unhelpful but Akane would be touched by, specially pre-relationship/the very start.
ALSO! Their height difference is cute! It would start with Akane being taller:
And if grown up Kou REALLY is the same as Teru, he'll become a little taller than Akane.
The idea of Akane either in complete denial his Kou looks like Teru when he gets older, or him genuinely being so in tune with every detail of his features that he is the only one who sees Kou as completely different and far more beautiful than Teru are equally funny to me.