(it's canon that Shouto started writing letters to his mom after the sports festival so)
(also warning tododeku lmao)
Shouto frequently writes to his mom about how he may or may not be in love with Deku; the letters act as his awkward documented experiences of falling in love, and Rei gets to read about all of it. When Fuyumi and Natsuo visit her hospital room, she sometimes shows them the letters.