Jean is really surprised by how close he and Bertolt have gotten over the year they've been in Pokemon world! He always kind of dismissed Bertolt back home (I head canon that he would have been a lot closer to Reiner) as one of the guys who were too creepy and clingy with their best friend (hello, pot, meet kettle. you will notice that you both are black). Like, Bertolt's best friend was always so larger than life and boisterous that Jean never bothered to get to know Bert.

He's come to realize that that was a mistake. Since getting to know him better here, Jean has found out that Bertolt is actually a pretty great guy; he's reliable, cleans up after himself, lets Jean boss him around in their tent, and they keep each other's secrets. Jean would hate for anyone to know how much he still misses Marco sometimes, and the fact that Bertolt keeps his confidence means a lot.

Jean wouldn't vocalize anything this way, since it would still feel like a betrayal to him, but Bertolt is one of his best friends now, and he's terribly torn about that. He wants Reiner to show up, so Bertolt can be happy, but is also really scared of losing him.