Robin and Lucina have the weirdest inside jokes. This all happened just because they would still be up at like 2 am and after they got comfortable with one another, they would get into whatever topic. And then Robin would over explaining a joke so Lucina would get it, and eventually it just stuck. Chrom isn’t sure why Robin saying “it was the eels” sent Lucina into a giggle fit but he does appreciate that they seem to be loosening up.