If I had to guess, I'd say it was some sort of gameplay/story thing. To make you feel like the Dawn Brigade is a really ragtag group fighting an enemy they are tragically underprepared to do battle with, but gain more powerful allies along the way to bolster their forces and stand a chance. Except it's not *just* the Dawn Brigade proper that stink in Micaiah's chapter--at least half the units that join are a complete joke like Fiona or Meg or are never around like Tauroneo or Tormod. Nailah, her husband and Sothe are hard-carrying these losers.
The Dawn Brigade is struggling along despite the fact that Daein's personal terminator, the Black Knight occasionally shows up to turn their enemies into a fine red mist.
Also, Fiona is almost comically awful when you consider her lineage. She's the daughter of Lavenga one of the Four Great Riders. The Riders have had members like Gawain/Greil, Bryce, Tauroneo, Petrine, Bertram, and the Black fucking Knight, and you meet the daughter of one of the members that didn't appear in PoR, and who is she? Some absolute waste of a cavalier, who I don't even remember talking after her introduction chapter.
To quote the wiki about how great this daughter of a legendary warrior is, "However, despite her unrivaled potential, she arrives at a low level with absolutely pitiful base stats and extremely limited availability." Fucking Renning is better, and he just gets randomly tossed into your party near the endgame to wrap up the whole, "Who the fuck is Bertram?" thing from Path of Radiance.
If they really wanted to do something crazy with her, why not make her an anti-Ike? She was the child of one of Daein's original riders, just like Ike, but you can show how their upbringings changed them. Ike is effectively stateless but winds up helping Elincia because its the right thing to do, and his whole shtick is about the strength of character and ability is more important than any lordly title. You could have Fiona be a hardcore nationalist, who unlike Ike, grew up knowing exactly who her father was, and just what kind of status that should afford her. And just like Ike she could be a character who has massive combat potential because they were both trained by two of Daein's best warriors from childhood. You could go in a million different directions with her, and at best she's a cardboard cut out of a filler unit.
I loved PoR, in large part because it eschewed the franchise standard, have a war that somehow escalates to you killing Satan, but for me while there was plenty of fun stuff in it, RD drops the ball. Especially since stuff like the Blood Pacts are such a writing cop out, letting characters do horrible stuff, that we can't judge them for because they didn't actually make any of their own choices. Also, I frankly have a hard time replaying it, because than I have to replay the Dawn Brigade chapters, which just give me a migraine.
