Darkshine and Genos are on good terms with one another with Darkshine showing respect and politeness towards Genos during the Alien Invasion, even trying to make sure that the cyborg calms down so that Tatsumaki does not harm him. Genos has acknowledged Darkshine as an S-Class with true strength after witnessing the hero defeat Carnage Kabuto in fifteen minutes during a fighting simulation.
During the initial attacks against the Monster Association, Darkshine and Mizuki seemed to have a teacher-student esque relationship. When Mizuki got overwhelmed by monsters, Darkshine assisted her by killing the monsters and complimenting her muscles. Darkshine also told Mizuki to not overburden her muscles and Mizuki told Darkshine that she would try to reserve stamina like him. Mizuki calls Darkshine "senpai" and was impressed at Darkshine's nonchalant behavior towards monsters attacking him. Mizuki is baffled at Darkshine's sheer strength and wonders what training he went through in order to get so strong.
Although there is never direct interaction between the two, Darkshine expresses a little bit of peculiarity at Saitama. In the manga, Darkshine was surprised when Saitama came out of the Boros' spaceship and was one of the only S-Class heroes present to recognize this (alongside Tatsumaki and Genos).
During the fight against Garou, Darkshine again showed peculiarity towards Saitama after he interrupted the hero's Superalloy Butterfly due to being in the way.
Despite Darkshine's immense physical strength and prowess, he still acknowledges and respects King's title of the "Strongest Man on Earth" (which, ironically, could've possibly been Darkshine's title if not for King's status). Darkshine sees King as stronger than even Tatsumaki as he tried to calm her down after she berated the "Strongest Man on Earth". Darkshine also says that King is one of the three S-Class heroes not to anger, comparing the hero to himself and Tatsumaki.
Darkshine has great respect for Bang and believes that, along with himself, he is the best at hand-to-hand combat among the S-Class heroes. Darkshine acknowledges Bang's superior fighting skill and ability after the martial artist defeated him in a one-sided sparring match.
The two appear to be on good terms with one another, with Puri-Puri Prisoner nicknaming Darkshine 'Darky'. 
During the Monster Association arc, Puri-Puri Prisoner searched for Darkshine. When Darkshine was distraught, Puri-Puri Prisoner released his full power Angel Rush which Darkshine withstood. This, along with further motivation, allowed Darkshine to get out of his state of self-doubt. Despite their closeness, Darkshine is afraid of Puri-Puri Prisoner falling for him and becoming one of his "boys."
While there was never a true interaction between the two, Darkshine and Flash are known to be a dangerous combo in battle due to the former's strength and the latter's speed. Darkshine has a great deal of respect for Flash, believing that if Flash were to fight pre-monster form Garou, Flash would win.
The two and Puri-Puri Prisoner would stand together against Garou.
Darkshine is initially excited to fight Garou and potentially use 100% of his power. After seeing a demonstration of Garou's power, he offers the chance for Garou to atone for his sins and "reforge the muscles of his mind" to become a true hero at the Hero Association, stating that it would be a waste for the hero hunter's premium muscles to rot away in the Monster Association. As their fight continues, he realizes Garou's unbelievable power and its unending growth and gradually becomes more afraid at the concept of losing to him. Once the Monster Association headquarters starts to collapse at the hands of Tatsumaki, he urges Garou to flee stating that he will be crushed if he doesn't, showing that he holds a level of compassion and respect for the hero hunter despite being his enemy.