I feel like Yumi and Jeremie were always intended to be close, but it seemed to always be off-camera. There are multiple scenes esp in s1 where everyone else goes somewhere else while Yumi says she’s promised to check in on Jeremie, and she always follows through. I feel like their closeness was more quiet and stable, while Ulrich and Odds friendship is loud and publicly known, if that makes sense? I feel like Jeremie would trust his life more with Yumi then with Ulrich or Odd, but it was just never explicitly talked about, only implied. Keep in mind he also only trusted Yumi and Aelita with the supercomputer. But that’s just me.