Mokuba + Rebecca
This pairing is brought to us by the anime and that alone, since Rebecca doesn’t exist in the manga. A very minor pairing in the fandom and it’s something I usually see in fanfics as a side-pairing. The problem is that Rebecca is a very minor character, especially for dub watchers, and not a lot of people are fond of her character either.
Truthfully, there isn’t any evidence for this pairing in the canon. It’s all in the fan’s imagination here. They don’t meet each other until well into later parts of the anime. They barely even get a chance to interact with each other. So why would anyone pair them up? It’s probably because they’re the only ones around each other’s age.
I personally don’t see why it’s not possible. They both frequent the Duel Monsters circle and are acquainted with Yugi. We’ll never know though.