Person A, the royal heir/ess, already weak of constitution, fell ill and have no more future. Person B just got adopted to replace them, even though they’re still alive.

Person A trying to get something too high for them in some store and Person B lifts them up instead to simply head it to them.

Person A is an adventurer. One day, they enter some ancient and lost yet sacred place, and since then, they're haunted by Person B, a divine spirit who thinks they're the reincarnation of someone they once knew (maybe a lover, maybe a friend) — so they just keep listening their sassy or amused comments in serious situations, they get advices from them, and built a strange relationship. Person A don't even know if they've gone crazy or it is real at some point.

Teaching the other how to swim.

Imagine your ocs/otp trying to help each other putting on difficult to wear clothes and both struggling.

imagine your ocs doing for the first time origamis... Or at least, trying to.

Being a nymph who married a god/dess is rather a difficult situation, especially with the looks others gives you.

As a hybrid, your character, somehow born of a mermaid and a human, can't properly swim or breathe underwater.