They seem to be good friends and are often seen together. The two heroes-in-training were paired together during the Battle Trial, and the U.S.J. Incident, showing to work decently well together. Kyoka was especially helpful to Denki at the U.S.J., helping him when he thought that he couldn't fight alongside Kyoka and Momo without hurting them, but she kicks him into one of the villains, claiming that she can use him as a "human stun-gun". Her quick thinking was very helpful at that moment, as it was very effective against the villains and it gave Momo time to create a protective blanket for Kyoka and herself to use.
Though the two seem to work well together, they tend to argue quite a lot, although this is generally viewed as friendly banter rather than genuine quarreling. Kyoka will often pick on Denki by saying rude things that will darken his mood as a form of comic relief. She also, more so than everyone else in Class 1-A, happens to enjoy Denki's brainless state and is often seen in the background laughing at it with the other characters. Despite all this, or perhaps as a result of it, Denki does consider Kyoka a great friend and she views him in the same way.
Denki is very supportive of all his friends, and this includes Kyoka. When Kyoka shows her insecurities around her music hobby, he compliments her, encouraging her to embrace her hobbies publicly.
Kyoka was surprised to see him at I-Island before Denki told her that he and Minoru applied to work as waiters since it meant a free trip to the island. When Kyoka showed up in a fancy dress, Denki and Minoru were "complimenting" her, which causes her to shoot vibrations with her jacks into their heads, showing that she didn't like the "compliment". When Wolfram and his gang were invading the island, Denki said that they should just sit and wait until the villains are done. Kyoka got up to ask if Denki was okay with that, to which he responded that they had no choice. Kyoka later asked if they should at least try to stop them. The two were seen next to each other hiding when the other members of Wolfram's gang went searching for them. When the group went to the one hundred-thirtieth floor, they saw I-Island Security Bots, later realizing that they were trying to capture them. After Momo made an insulator sheet with her Quirk. Denki used an Indiscriminate Shock 1.3 Million Volts on the Security Drones, but to no effect before upping the voltage to two million, realizing what he was about to do despite Kyoka's pleading not to as he would short-circuit faster, which he did. Later, when the one hundred thirty-eighth floor was surrounded by more Security Drones, Tenya let Denki down from his back before he was being held in Kyoka's arms. When the security system was rest, the two along with Tenya, Momo and Minoru were freed, and then when Wolfram used the Quirk Amplification Device, he and the others regrouped with the rest. Kyoka and Ochaco were seen holding him while he was still in his mindless state from the volts he released, the former later seen still holding him as the fight with Izuku and All Might against Wolfram raged on, she cheered them on as she was still holding him.
When Nine and his group were invading Nabu Island while on the hunt for Katsuma Shimano's Cell Activation Quirk, the two helped with reinforcements and their fight with Mummy, Kyoka asking if they were fine after the fight. After Nine retreated, Kyoka looked on as Denki and Momo were making emergency supplies for the people to supply them with what they needed, overusing their Quirks; Denki started to short-circuit as his brain was starting to fry from overusing too much of his electricity. When Class 1-A was reflecting on the villains they fought, Kyoka told the class that Denki and Momo had used up too much of their power already.
When Denki begins to panic and fear for his life during the raid on the Gunga Mountain Villa, Midnight tells him to think of the person who matters most to him, and Denki's focus centers on Kyoka, who encourages him and tells him to stay strong. At this point, Kyoka seems to be one of Denki's closest friends, and it's likely that Kyoka knows Denki thought about her following Midnight's advice because, as her earphone jacks were plugged into the ground, she spoke up right when Denki looked back at her.
Since they happened to be seated behind one another in class, they became close friends. They often make comments to each other. When the students are together, they are almost always standing or sitting near each other, even when they are out of class. They appear together especially often during comedic scenes.
They chose to work together along with Katsuki for the Provisional Hero License Exam. After Seiji from Shiketsu High School defeats Eijiro and Katsuki, Denki stands up against Seiji to help them. He taunts Denki by stepping on Eijiro, which greatly upsets him. He ends up defeating his opponent, saving Katsuki and Eijiro.
Overall, they have very similar personalities, both being very positive, friendly, and likable, although Eijiro is somewhat more boisterous compared to Denki, who is more mischievous. Their similarities personality-wise make them able to get along extremely well. They are also similar in the fact that they both get along well with Katsuki, although Eijiro spends a little more time with him then Denki does.
Proof of their friendship is that Denki is the very first one to congratulate Eijiro for his hero debut defeating a villain alone while he was patrolling on his first day of Hero Work-Studies at the Fat Gum Agency, excitedly showing everyone the news of his feat.
Denki initially did not like Katsuki, finding him to be an aggressive bully, but Denki has warmed up to him over time, as he feels comfortable openly teasing Katsuki and trying to converse with him. An example of Denki teasing Katsuki is on the bus ride to U.S.J. while Katsuki angrily insulted him.
Denki was intimidated by Katsuki in Final Exams Arc but agreed with his point of mastering his Quirk. As time goes on, it seems Denki has become comfortable around him and considers him a friend despite his violent tendencies and sees his determination to become a Hero is stronger than most. He often hangs around Katsuki and Eijiro. He even got away with calling Katsuki, “Kacchan”, an affectionate nickname that Izuku addresses him by.
During the Provisional Hero License Exam, it is shown that Denki sees Katsuki as a friend when he saves him and Eijiro from Seiji's Quirk and respects him enough to stand up for him. He goes up against a Seiji in order to save his friends.
Due to their shared perverted interest in girls, Denki and Minoru occasionally hang out with each other. They both once played a prank on the Class 1-A girls during the U.A. Sports Festival, where they tricked them into wearing cheerleading outfits for their own personal amusement. Together, they tricked Izuku to help persuade Shota Aizawa to give them permission to use the school pool just to see the girls in their bikinis. Although they were both disappointed to find the girls wearing school swimsuits, Minoru was still appreciative that the girls were wearing swimsuits while Denki insisted that he should get some standards.
Denki and Minoru differ in the fact that Denki respects the girls' privacy, and despite being a little perverted, he knows not to take things too far. On one occasion he tells Minoru that he's crossing the line when he reveals his plans to climb the wall to spy on the girls' side of the hot springs. He was also shocked when a peephole to the girls' locker room was discovered, while Minoru had no problems with invading the girls' privacy and saw the peephole as a blessing. They discussed potential Ultimate Moves that they might develop. Denki was jealous that Minoru got a higher grade than him at the exams which meant he was more intelligent.
Denki and Mina are often seen together and are considered to be friends, although Denki is not seen as frequently with Mina as he is with Eijiro and his other friends. They somewhat bonded due to them being the worst students in class in terms of grades.
They were partners during the exam before the training camp.
Denki and Hanta appear to also be friends, often hanging around and talking to each other, indicating a friendship. They share a very social personality, tending to be among some of the louder and more extroverted boys of the class. They enjoy making jokes with each other, and appear to be very comfortable around one another.
Mashirao sits in front of Denki in class and the two are often pictured talking to each other in the background. Denki seems to have a fixation on Mashirao's tail and often plays with it and pets the hair at the end of it. The two of them appear to have become friends as time goes on, Denki sometimes confiding in him with certain things.
Denki and Momo had encountered several obstacles together, mostly during the U.S.J. attack where they fought against villains along with Kyoka. In addition, they were also together in Team Todoroki during the Sports Festival. Later on, seeing her fellows having educational difficulties, Momo suggested helping Denki and Mina with their revision. Denki and friends go to the study session, and during their time there their friendship intensifies. After the exam Denki thanks Momo for her help.
It is also seen later in the manga where Denki and Momo were chitchatting and laughing on the sofa, reflecting on the fact that they are good friends.
During the U.A. School Festival event, Denki and Fumikage are shown interacting more with each other. They were members of the A Band, both guitarists for the class's show. Both lads spent hours practicing and learning how to master several techniques and chords of this musical instrument. Furthermore, Fumikage tells Denki that he is a magnificent friend and encourages him to fight in the battle.
Fumikage watches Denki charge forward and thinks back to when they practiced together, how he hurt himself strumming the guitar and he offered for the two of them to take a break. Denki said they should keep going a little longer because he wants to help out Kyoka anyway he can with his performance. From this statement, Fumikage deduces that Denki is a man who cares deeply for his friends from the bottom of his heart.
Denki and Izuku are on good terms and they're sometimes seen talking together. Denki also considers Izuku as "very diligent". He is also seen to praise Izuku when the latter saves All Might by using for the first time, his Shoot Style during training to create Ultimate Moves. Like Minoru, Denki has been shown to have been slightly jealous of Izuku gaining affections of females like Camie, who was actually Himiko Toga in disguise, and Ochaco Uraraka, but he still views him as a good friend.
After Paranormal Liberation War Denki and the rest of Class 1-A receive letters from Izuku, in which he tells the truth about his Quirk and that All For One and Tomura Shigaraki are after him, so he must leave U.A. Denki is shown expressing shock and confusion in the sudden revelations.
He later joined in trying to bring Izuku back, despite the latter attempting to flee from them. Denki outright called him a friend and begged him to see the class has his best interests at heart. 
Denki and Ochaco appear to be on good terms, although they rarely interact.
Denki has been shown to express interest in her, attempting to ask her out during the Battle Training Arc, although she ignores him to congratulate Izuku.
Denki thinks that Ibara is beautiful though quite intimidating. Just before their match during the U.A. Sports Festival, he asks her if she wanted to grab something to eat later, and that he would be happy to console her after the match. Ibara proceeds to quickly defeat him in their match.
They also fight once more in the Joint Training Arc, where Denki mentions that she poses a serious threat to their team, and offers himself as a decoy in order to help his team counter her immense vine output.
They have not had many interactions together so far, as Denki talks to Hitoshi for the very first time in the Joint Training Battle. Despite Hitoshi's cold attitude, and his statement that he was not there to make friends, Denki makes an attempt to befriend him anyways. He tries to be friendly to Hitoshi, and he defends him when Katsuki describes him as a deadweight for the exercise due to his lack of experience.
They teamed up during the first round. Even though Hitoshi thinks he was too slow in capturing Jurota and apologizes to his teammates, Denki points out that he did a good job, and he tells Hitoshi that he's rooting for him to become a great hero someday.
Denki also comments on Hitoshi's appearance, saying that he has the face of a popular guy, implying that Denki thinks Hitoshi is good looking.
Since Seiji watched the U.A. Sports Festival, he was aware of Denki's Quirk being untamed and he taunts him by saying that he can't use his Quirk because it will also hurt Eijiro and Katsuki despite their deformed state. Denki took those insults personally especially when Seiji stepped on Eijiro's deformed body while taunting him. He ends up underestimating Denki which led to his defeat.