Mikuriya runs the research center where Naoto and Naoya spent their entire childhood. He basically raised them there—kept them isolated from their parents for fifteen years, which understandably made Naoto furious. The resentment stuck around even after they escaped. That said, the brothers didn't completely cut him off afterward. When he needed help in episode 14, they actually came through for him. Mikuriya sees himself as their guardian and protector from those years, though it's pretty clear the brothers have complicated feelings about that relationship.
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