I have this headcanon for Yuma that when he was a young child, he liked to pretend he was an adventurer just like his dad. He’d go on quests for ancient artifacts (technically just random objects like a rubber duck if he was pretending to be in an underwater ruin while in the bath). Usually, he’d play these games by himself, but sometimes Kotori would join in to help him escape from tomb traps or kill a monster Yuma missed or help unravel a mystery Yuma was having trouble solving. They would go everywhere from the sunken depths of Atlantis to the creepy underworld of various ancient civilations (like the Mayan underworld of Xibalba, for example).

And when his dad disappeared, he used his pretend games as a coping mechanism. Sometimes he’d even pretend he was on a quest to rescue his dad from aliens or something scarier.