I don't think they're just best friends. They could be, sure, but Hori purposely had them go hiking for Christmas! He purposely has them in couple's costumes! He had Bakugou give Kirishima his festival jacket! They're shown constantly sharing food! He has Kirishima sitting SHOTGUN in Bakugou's CAR.
Hori made the nature of their relationship VERY clear, but he did it in such a slow manner that editors either didn't catch on or didn't think it would be a big deal/think anyone would notice.
Seriously. The amount of official art with them together is NUTS. For a tertiary character (Kirishima) to get that much attention is wild.
