My Hero Academia has turned up the heat immensely in season 6 as the war between heroes and villains takes off. However, among the intensity, the series has managed to allude to something contrary to the violence ensuing between the Paranormal Liberation Front led by the newly-awakened Tomura Shigaraki, and the heroes desperately trying to crush a major threat.
While romance is nothing new to battle shōnen, it comes somewhat unexpected in the beginning stages of the war arc when Denki Kaminari (Hero Name: Chargebolt | Quirk: Electricity) took center-stage in a dangerous but valiant effort to ensure the invasion of the villains' base continues unimpeded. Worrying about Denki's safety is none other than Kyoka Jiro (Hero Name: Hearing Hero – Earphone Jack | Quirk: Earphone Jack), but since when have these two been romantically involved? Since when have they been a thing?
The Battlefield
While charging the villains' base of operations during the Gunga Mountain Villa Raid, Denki is incredibly flustered and can't focus on the task at hand due to being overcome by fear. The operation is incredibly dangerous, and they're charging into the very heart of the Paranormal Liberation Front in an all-out assault. The stakes are higher than they have ever been, and the heroes have even enlisted the help of youths in training in one of the most important operations ever carried out in the heroic age. His fear and anxiety are warranted, but greatly unhelpful to the operation. The pro Hero Midnight attempts to talk him down and inspire some level of confidence in him.
She tells Denki to think about the person who matters the most to him, and Kyoka Jiro is the person who comes to mind. The two are constantly together and have been so for a long time, but as supporting characters, the development of some kind of deeply intimate relationship, romantic or otherwise, has hardly been given much screentime. However, we already know that the relationships between students within classes are generally friendly, and on a general basis, each student considers each other student an ally on some level. For Denki to think of Kyoka at that moment is indicative of deeper feelings or a deeper level of intimacy between them.
Tsukkomi To Boke
Throughout My Hero Academia, Denki is presented as a loud but non-threatening presence in his class. Kyoka in particular considers Denki to be irritating and does not spare him any slurry of snarky or sarcastic comments that may come to mind. There are many Idiot/Wise Guy-coded interactions that take place between Kyoka and Denki, most of which leave Denki looking or sounding rather silly, and Kyoka somewhat irritated at first.
On some level, this dynamic is born from teasing on both ends of the relationship. In later stages of My Hero Academia, this dynamic changes a bit as Kyoka begins to thaw in her reactions to Denki, especially after the Hero Killer Arc of the second season. The Provisional Licence Exam Arc is where Kyoka and Denki's interactions get tinged with a slightly more amorous atmosphere. When the students of 1-A move into their dorm rooms and explore each other's rooms, Denki is visibly distraught at Kyoka's comment that his room looks like the one store in the mall she'd avoid.
When Kyoka's room is shown, Denki mentions the large number of musical instruments in her room. Music, while very evidently Kyoka's reason for living, is still a very touchy subject for her, and she gets embarrassed about her passion for it very easily. In a later arc, Denki, in conversation with Mina about how her dancing enabled her to access greater mobility in combat, talks about how Kyoka's huge passion for music probably has a similar level of combat potential.
The conversation makes Kyoka blush, and she implores Denki to drop the subject; however, it's clear that over the course of several seasons, Denki and Kyoka's relationship gradually shifts and changes into something else. Denki's assistance of Kyoka during the Festival Arc in the third season cemented some kind of relationship between the two, particularly in how Kyoka reacted to Denki's compliments of her musical talent.
Common Ground
One of the things that has been building over the course of the My Hero Academia narrative is how Denki and Kyoka have several things in common, even though it took some time for their relationship to really take. For one, both of them are inspired by rock or punk; and to that effect, the two actually share a costume designer. When Denki comes to learn more about Kyoka's music affinity, he is genuinely impressed by it, and this ready acceptance of this aspect of Kyoka's personality and lifestyle is part of what makes her warm up to him.
By the time the War arc rolls around, the two have already been through several tough situations and some great ones to match, and when the stakes are the highest they have ever been, the one person on Denki's mind is Kyoka Jiro. Whether this means they're an item is one thing, but there is no doubt that the two of them are very close.