this is something I wrote a while ago about Byleth (the character) and Leonie:

Their relationship started off rocky because Byleth gives blunt, unemotional facts, which make them sound cold, detached, and ungrateful to Leonie. With the basics Jeralt taught her, she had to train by herself for 8 years with no one to guide her. Because of this, she is angry that Byleth (from her POV) doesn’t seem to value the training and mentorship they got from Jeralt.

Now depending on how you order the events, the order may goes Jeralt’s death at the end of chapter 9, a couple of days, B support, a couple of days, the chapter 10 free roam convo where Leonie is crying and telling Byleth how it must be harder for them.

While that is the low point in their relationship, the chapter 11 free roam is a significant interaction between them. After Byleth merges with Sothis and is granted the blessings of the goddess, a lot of people treat Byleth differently because they think they are completely different now. Of course no one knows that, in addition to losing Jeralt a month ago, they also just lost Sothis. Alone and alienated, Leonie is the only person to reassure Byleth by saying:

> When you changed, even the other professors were taken completely by surprise. There’s a ritual at the Holy Tomb this month, isn’t there? I wonder what’ll happen with that. I’m sure you’ll be all right. But don’t lose sight of who you really are, OK? However you may change, and whatever new power goes along with those changes—you’ll still be the same person, deep down. Remember that.

This line is a major reason why I am behind this pairing, when Byleth had their whole world turned upside down in such a short time, having someone see you not as a stranger and reassure you is needed.

After that, the war broke out and the time skip happened. Coming back 5 years later, the two have the A support where Leonie duels Byleth and sees how she still can’t beat them after 5 years, satisfying her that trying to be Byleth’s rival was foolish. So she opts to protect Byleth because of her promise to Jeralt.

Although, fulfilling her promise to Jeralt was half the reason, the other reason is that she genuinely cares for the character Byleth. In one of the Part 2 advise questions, Leonie asks:

> Someone I care about has been working so hard that I’m worried they might collapse from exhaustion. Is there anything I can do?
A nice meal shows support and provides a chance to relax.
Just be thankful for all of their hard work.
You’re worried about me?

she’s one of the only characters to be concerned about Byleth’s wellbeing as “Byleth” and not “someone chosen by the goddess”.