Starting with how Bakugou has loved his quirk from the moment he got it cause it made him feel powerful and better, comparing it to how Kirishima started hating his own for a while cause the first thing he did with it was hurt himself. Going on with how Bakugou grew up being a bully and Kirishima fighting them - and how Bakugou was strong and feared while Kirishima was ignored and taken lightly because he was considered weak. How Bakugou’s whole understanding of the world/life was based on how he saw his place as above everyone else, how Kirishima always thought himself as lame and under all his peers. How Bakugou’s always had no problems yelling at the world his goal to become a hero and how he’s always had everyone’s support on it, how Kirishima’s always hidden it and how no one ever truly believed he could do it anyway
And more still, how Kirishima’s whole moral is based on Do Good and Be Fair while Bakugou’s just learning that caring is maybe something he should do now and again, how Kirishima wants to be a hero to save people and how Bakugou wants to be a hero to fight people, how Bakugou’s rarely seen without a scowl and how Kirishima’s default expression is a smile, how Kirishima’s an act-before-thinking person while Bakugou overthinks everything, Kirishima’s simple-minded and Bakugou’s only slightly short of a genius, how Kirishima’s friends with everyone and how Bakugou managing to get himself five friends is considerable a miracle
How Bakugou’s quirk is the perfect attack and Kirishima’s the perfect defense, and how Bakugou’s got the flashiest quirk in the class and Kirishima the least flashy one. How Bakugou’s pivotal moment in his growth right before entering UA was about getting attacked and being unable to save himself/being forced to accept help he didn’t want while Kirishima’s was seeing someone getting attacked and being unable to help them - and how those themes keep on repeating themselves for both of them through their whole personal arcs
The more I think about it the more these two are each other’s perfect opposite. They’re complementary. And they’re best friends.