Tiamat's a fascinating case because she's technically a Dragon King but operates more like a shadow ruler in the DxD world. She's got this iconic rivalry with Ddraig and Albion, the two Heavenly Dragons, which sets up her as a peer from a legendary era. It's not front-and-center in the main plot, but it informs her immense power level and ancient status. She's got a very neutral, almost detached relationship with the core cast. She respects Sirzechs and the other Maou because she's a businesswoman at heart, dealing in information and magical artifacts. I'd say her most direct link is to the Gremory group as a sometimes-ally, sometimes-client. She supplied Issei with the Wyverns, which is a huge deal, but it feels transactional. There's no deep emotional bond there, which makes her unique in a series built on intense personal connections.
I keep hoping we'll see more of her backstory with the other Dragon Kings, like Yu-Long and Vritra. The novels hint at these ancient dynamics but never fully explore them. It's that classic powerful being who watches the new generation from the sidelines, occasionally nudging things with a resource or a piece of advice. Her relationship with Ophis is another loose thread—both are dragon-type deities of immense power, but their interactions are minimal. She's a character built for potential spin-offs, honestly.