I have a darksteelfoil hat theory that Square Enix really wanted Yda to be a hot-headed, revenge-driven youth hells-bent on reclaiming Ala Mhigo for Stormblood, but given that Yda was hanging in the balance of 20 years of half-fulfilled plot threads going from fighting in 1557 up through being unexpectedly aged a further 5 years by the jump to ARR, having the Lady in Red be Yda, pushing (if not over) 40 years old (with barely any character development or personal growth since the fall of Midgardsormr back in '62) didn't work very well, so they copped right out of it and put her back in the frame of a 20-something off to kick some Imperial ass.