First off, I don’t mean romantic relationships exclusively. It can be any sort of bond (or hatred) between two or more of the Battleborn.
My personal favorite is with Boldur and Thorn. Reason: Boldur cares for her like she is his own granddaughter. I’ve just been touched by how much he cares for her. When he sees her on the enemy team he calls out that he doesn’t want to fight her and if he kills her you can hear him cry. Thorn shouts out, “I get to kill my grandfather” when she sees him and that’s a little less caring than Boldur but I stand by what I said. Also when I read the last two lines of Boldur’s lore explaining how they met, I clutched my chest and said, “Ow, right in my feelings.”