Felix isn't used to expressing gratitude or appreciation in words. He tries to do it in other ways, but it's difficult to tell how much of the sentiment comes across when he's...well, deliberately avoiding sentiment.

But he's sure that if he tried to tell these things to the professor in person, he would only fumble his words and end up sounding pathetic. He doesn't want to send a text message, though...

So one day, Byleth will find an envelope with his name written on it in familiar handwriting waiting for him on the kitchen table in the house he shares with Dimitri and Dedue. Inside is a letter.

Professor,

In speaking with Seteth, who recently arrived, I had reason to consider how my life would differ had you cut a path for yourself that diverged from mine. I know for certain that I wouldn't be who I am without your guidance. You pushed me to become better, not just in battle but in body, mind, and spirit.

It occurred to me that perhaps I have not expressed that to you enough. So choose a day, and for that day you and I will do whatever you like. I'll even put aside my blade for the day, if you decide you would rather not spar. (Just this once.)

Felix