I have a theory that Byleth inherited the Crest of Seiros from Jeralt, but it could not manifest due to the Crest of Flames' dominance. I believe it would resurface after the events of Crimson Flower
The Jeritza S-Support art, with him holding Seiros's sword, really backs this up
Leonie Pinelli meets Jeralt Eisner in Sauin Village, circa 1174 (colorized)
#FireEmblemThreeHouses #Jeralt #Leonie #Persona4Arena #Persona3 #AkihikoSanada #Persona4 #ChieSatonaka
Jeritza likely lied about his age, though, and Jeralt lied about Byleth's age (saying they were born after he left the monastery), so yeah.
🧐