KIRIBAKU SHIP DEVELOPMENT
I love many BNHA ships but Kiribaku holds a special place in my heart since it has so much positive canon material and I am always a sucker for the "Best friends who fall in love" trope since I enjoy slow burn with character growth. However, even taking the romantic possibility out of the equation, I just love that their relationship is so well developed in the story with subtle changes in their behavior all throughout as they go from 1. strangers who don't think much of each other to 2. reluctant allies who need to work together to complete certain tasks to 3. sort of friends with Kirishima pushing for a friendship to 4. best friends with an equal balance of respect and appreciation for each other. Below I tried to break down their relationship as it develops throughout the manga so...
WARNING: This is a long post and MAY contain spoilers for non-manga readers and anyone who has not seen the BNHA movie (not by much though).
Back at the beginning of the series, Kirishima used to think that Bakugou was a terrible person just like the rest of class 1-a, especially due to how he would treat Izuku. He was all like "That Bakugou is crazy and fights dirty" while Bakugou didn't even care about his existence yet. The only thing on Bakugou's mind at the time was beating Izuku and proving his superiority over everyone else. Thus, Bakugou was definitely not looking for friends as the only people he was used to having around him were "followers" as shown during his middle school times.
Then during the USJ arc, they automatically started working together to fight the villains as they were right next to each other during the initial attack. This right away showed how synchronized they were as a team due to their personalities and compatible quirks. So from the get go, they just seemed to flow well together.
Afterwards, they were sent off in the portal together giving them more time to fight villains side by side alone to get a feel of each others' personalities. During that time, you can slowly see Kirishima change his perspective of Bakugou from "This guy is crazy" to "Huh. This guy may not be that bad. He seems pretty cool right now".
This scene may seem minute but it is the moment that ignites Kirishima's interest in getting to know more about Bakugou. Kirishima basically starts to become more interested in Bakugou as a person as he starts to recognize his positive qualities despite his original assessment of Bakugou's personality.
Kirishima can't help but start to find Bakugou "manly" even after he tells off the other classes at the beginning of the Sports Festival arc. Even despite Kaminari's warning of "Don't let him play you" lol And this is just great because it shows that Kirishima was still somewhat hesitant about liking Bakugou as a person but was willing to give him a chance.
Kirishima begins to want to hang out with Bakugou and get on his good side as shown by the interactions above and some of the interactions at the beginning of the Sports Festival. Bakugou still doesn't think much of him but he let's him be for the most part. You also start to see Kirishima more impressed with Bakugou than before.
As the Sports Festival progresses, Kirishima finally gets his opportunity to prove his skill in Bakugou's eyes while also appealing to Bakugou's desire to win against Izuku. This moment shows an understanding of Bakugou's personality as he knows how to get Bakugou to agree with him despite Bakugou not thinking too much of him yet. It also shows a shift in Kirishima's preference of Izuku since now he would much prefer to be in Bakugou's team as he didn't even try to get in Izuku's team and barely tried to get into Todoroki's team based on his commentary.
THAT moment is when their friendship is truly born as Bakugou starts to acknowledge him and call him "Kirishima" afterwards as a sign of respect. During the match, you can see their dynamics in action with Kirishima trying to calm down Bakugou as Bakugou starts to lose his cool with opposing teams lol So basically the beginning of a beautiful friendship.
The two are eventually paired off against each other and Bakugou wins but he STILL respects him as an opponent. He even comments on how difficult it is to get past Kirishima's defenses which is why he basically lets him reach his quirk limit to win. The fight is short but pretty funny because you can see them kind of teasing each other all throughout like good friends.
After the Sports Festival, the two are often seen around each other hanging out with Kirishima definitely feeling more comfortable around Bakugou to the point that he easily jokes around him and teases him. Slowly, through Kirishima's influence as he basically gets along with everyone in class 1-a, you can see more of the other students also feel more comfortable around Bakugou, especially the ones who were on his team at the Sports Festival.
Characters like Kaminari, Sero, Mina, etc. Sero is one that automatically attaches himself to Bakugou's group soon after the Sports Festival as he is shown comfortably teasing Bakugou for his haircut with Kirishima.
By the time the Summer Training Camp Arc happens, the audience already has some understanding of their friendship but it's still not in full display yet. It's more subtle and in the background than anything. During the summer camp though, you can finally see how much Bakugou means to Kirishima as a friend and as a fellow student after the villain attack that results in his kidnapping.
He feels guilty that he couldn't help his friend so he tries to convince the others to help him save Bakugou despite that being illegal. He only manages to convince a select few, two of which were already there when Bakugou was kidnapped and felt responsible too (Todoroki and Izuku) but the fact that he felt that much about it shows how much he cares.
In part, knowing Kirishima's character and motivations, you can tell that he wants to save Bakugou to prove to himself that he can be a true hero but you can also tell that saving Bakugou means more to him than that since Bakugou is not just "some guy" but his close friend.
During the entire rescue arc, Kirishima is all about doing his best to save Bakugou alongside the others to the point that he buys expensive night camera lenses to spy on the warehouse and chooses the disguises for the others (which were pretty funny disguises that make you wonder why Kirishima has such terrible fashion sense but can choose decent outfits for others XD).
Later on, Bakugou pays him back the money for the camera as a form of thanks for saving him. If BNHA Smash is anything to go by, Bakugou apparently did ALL kinds of odd jobs to get that money as quickly as possible (i.e. Nude modeling, dress like a bumble bee, bake cakes, etc.) 🤣
During the rescue arc, the most Kiribaku moment ever occurs in which Izuku comes up with the "Rescue Bakugou Without Confronting Villains" plan that ends up working ONLY because of how Bakugou views Kirishima as a friend and equal by that point. This is an important moment of development from Bakugou's side as it shows that he holds Kirishima in high regard and does not feel like Kirishima saving him is him looking down on Bakugou's own skill. This is significant since THIS specific detail is one of the factors that causes Bakugou to react in anger towards characters like Izuku and Todoroki most of the time: his belief that they are looking down on him and consider him inferior. So him accepting Kirishima's hand basically confirmed that their relationship is mutual as they both consider the other a friend without fear of judgment.
After this arc, there are countless other subtle moments throughout the series that show how close they are such as:
1. Bakugou tutoring him and going on study dates (a lot of that in the light novels). Also, the fact that Kirishima clearly wanted Bakugou to tutor him from the beginning since he never seeked out Momo like the others when he saw his grade.
2. The BNHA movie moments (assuming they are canon) which happened before the summer arc and add MORE development and insight into their relationship (i.e. Bakugou inviting Kirishima to i-Expo, sharing a room, Kirishima buying them suits, etc.)
3. Kirishima constantly complimenting Bakugou but also making sure he's not too rude to others (Literally them every panel in the manga nowadays lol)
4. Kirishima apparently helping Bakugou move all his stuff to his dorm and then...
5. Kirishima knowing that Bakugou went to sleep at 8 pm when showing off rooms hinting that they were hanging out prior to that
6. Bakugou's motivational speech to Kirishima that motivated him to push himself harder to create the "unbreakable" form/also basically a compliment from Bakugou to Kirishima.
7. Kirishima implying that he and Bakugou spar together sometimes (i.e. Bakugou asking to test his technique on him and Kirishima being fine with that while the others are scared lol)
And don't get me started on all the BNHA SMASH moments! Seriously, those are not canon at all but there are many. And Kirishima is extra affectionate in those 🤣 He has the power to make Bakugou do pretty much anything there.
To conclude this long post, whether as brotp or otp, the Kiribaku relationship development is undeniable. Just like the character growth they both experienced due to each other. Now, just because they help each other grow does not mean they are the only ones there for each other. Kirishima is not the ONLY one that understands Bakugou in the story nor is he his only friend or person he respects since Bakugou has many other people he respects and calls by name (i.e. All Might, Aizawa, Uraraka, etc.). Likewise, Bakugou is not the ONLY person Kirishima admires or considers a close friend (i.e. He has Kaminari, TetsuTetsu, etc.).
However, they clearly mean a lot to each other and help each other develop where they need to. Kirishima helps Bakugou open up to others more and Bakugou helps Kirishima believe in himself and his ability to be a hero. And that is enough for them. 😁