On the surface, Idia and Riddle seem like complete opposites, but I honestly think they're one of the most compatible pairings in TWST because of how many parallels they share.
Their biggest parallel is their upbringing. Both of them grew up isolated, trapped by other people's expectations, and desperately wanting something as simple as freedom and genuine friends. Those shared experiences give them a foundation for understanding each other in a way very few people could.
They also think in remarkably similar ways. The cooking event is a perfect example: both of them got caught up trying to follow the recipes with absolute precision until the ghosts pointed out that the instructions were meant to be guidelines, not strict rules. Riddle also praised the programming books Idia created in his dream for being so clear and organized. Riddle's way of thinking seems perfectly suited to working alongside Idia. They're both incredibly intelligent, and intelligence can often be isolating. For someone like Idia, who rarely finds people who can keep up with him intellectually, it must be so exciting to work with someone like Riddle. You can already see that during Book 7, Idia seemed genuinely happy and energized when Riddle picked up coding so quickly.
They also share a love of animals. During the Nightmare Before Christmas event, both of them showed they understand how to approach animals and explained it well to others. The difference is that Idia gets a little too excited in practice and ends up scaring them away. I love imagining Riddle introducing him to the Heartslabyul hedgehogs and gently teaching him how to approach them. It's such a sweet image.
More than anything, though, they're both people who desperately need love, reassurance, and affection. They're guarded in very different ways, but underneath those walls they're both incredibly loving, arguably more than most characters. That's why I think they'd fit together so naturally. They would love each other wholeheartedly, which is exactly what both of them need.
Their love languages complement each other beautifully, too. Idia needs to hear that he's loved, and Riddle is someone who's honest and direct about his feelings. I can easily imagine him being incredibly gentle and openly affectionate with the person he loves. On the other hand, Riddle clearly needs physical affection to feel reassured. Looking at how affectionate Idia is with animals, I can't help but think that while he's awkward around strangers, he's probably extremely physically affectionate with someone he completely trusts, someone he considers his safe person. A bit like a cat, affectionate but very selective about it.
The best part is that neither of them would ever have to wonder whether they're loved enough. Because they're both such deeply loving people, neither would end up feeling like they're the one who cares more. They'd constantly reassure each other, and for two people who've spent most of their lives feeling alone and misunderstood, I think that's exactly what they deserve.
