Honestly, I think Zeoticus gets unfairly overlooked because he dies so early. His actual influence is way more structural than it seems at first glance. He’s not just a dead dad trope to motivate Rias. His decisions before the story even starts are what create the entire premise. By arranging the engagement between Rias and Riser, he sets up the central conflict of the first major arc. It’s a classic noble family power move, linking the Gremory and Phenex houses, but it completely ignores what Rias wants. That pressure is what forces her to act independently, form her own peerage, and eventually fight for her freedom. Without that arranged marriage hanging over her, she might have just been another high-ranking devil heiress playing by the rules.
Also, his legacy defines a lot of Rias’s internal struggles. She clearly respects her family and their status, but she’s also rebelling against the cold, transactional aspects of devil society that her father embodied through that engagement. You see her trying to balance being a proud Gremory with being her own person, and that tension starts with him. He represents the old guard—the part of devil aristocracy that sees marriage as a political tool and children as assets. His actions, even posthumously, force Rias, and by extension Issei, to challenge that system head-on. So in a weird way, he’s a catalyst for change by being the embodiment of the tradition they need to overcome.