In the 'Danmachi' universe, Erebus is such a fascinating wrench thrown into the godly and mortal dynamics. His relationship with the main characters is less about direct, constant antagonism and more about being an architect of chaos. He doesn't fight Bell Cranel himself; he sets the stage and pulls the strings so others are forced to the brink.
For Bell, Erebus is a distant, almost incomprehensible threat—a god who views mortals as pieces in an intricate, sadistic play to create 'heroes' through overwhelming despair. His masterstroke was orchestrating the entire Knossos and Juggernaut ordeal, pushing Bell to his absolute limits not out of personal hatred, but from a twisted desire to see a beautiful tragedy unfold. With Hestia, it's pure ideological opposition. She represents the familia bonds and genuine care he finds boring; he actively works to undermine that warmth with his cold, theatrical schemes.
His most complex ties might be with Freya and Ouranos. He respects Freya's power and chaos, seeing a kindred spirit in her capriciousness, but even she becomes a piece in his game. With Ouranos, it's a battle of philosophies: order and stability versus the beautiful, destructive drama Erebus craves. He’s not just a villain; he’s a director, and the main characters are his unwilling, struggling cast.