Rapt and Moyuru are Soga's best friends, having a strong feeling of loyalty between them. Almost always he is usually with them, and they usually commit their misdeeds together. Despite his violent and antisocial nature, Soga cares towards them, such as when he went to face Kuin alone and warned her to leave his friends out of their conflict.
At first, Soga hated Koichi, thinking he was another one of those people who mock and laugh at him of him. When Koichi accidentally spilled his juice, Soga demands him humiliate himself as an apology, to which he refuse and flees, although later Soga would beat him up.
When he and his friends assault Kazuho, Koichi and Knuckleduster defeat them. In revenge, Soga acquires Trigger and injects it, violently attacking Koichi, although he was stopped by Shota Aizawa before he did any damage.
Their relationship begins to improve after Koichi risked his life to protect his from Stendhal. Although he does not recognize it, he is grateful for it, and even begins to show certain signs that he also cares for him.
When Soga mets Kazuho in person, under his Pop Step persona, he and his friends made fun of her and tried to sexually assault her, which is why Kazuho hates him. After the incident with Stendhal, she treated his injuries and their relationship improves slightly, although unlike Koichi, she finds it harder to accept Soga, as she still does not forgive him for what he tried to do to her and does not hesitate to insult him. Soga, on the other hand, simply ignores her.
After Knuckleduster gave him a beating while helping Koichi and Kazuho, Soga hated him and wanted to take revenge on him. But after he saved his life when Stendhal tried to assassinate him, their relationship changes completely.
After being saved, he's hired by Knuckleduster to look into the whereabouts of his daughter and Kuin's host, Tamao Oguro, effectively making him Knuckleduster's informant. After Tamao is freed from Kuin's control, he and his friends watch over her bedside whenever Knuckleduster is out.
While he was occupying Tamao's body, Kuin sends Stendhal to kill Soga and his friends, although she tries to manipulate Soga so that he injects a dose of Trigger to have a chance against the murderous Vigilante. But when Soga refuses, Kuin injects the drug to him by force, turning it into a monster.
After being saved by the Naruhata Vigilantes, Soga helps Knuckleduster to find Kuin, and when Tamao is released from Kuin's control, Soga and his friends take care of her in the hospital. Soga seems to get along with Tamao Oguro, sharing a passion for music. Soga tunes her guitar and encourages Tamao to play it, which causes Rapt and Moyuru to tease him and call him sweetheart, to his annoyance.