+Def/-Res Eirika (Hello?)

I believe I have the same one, she's basically neutral. All I did was give her Rally Def and she does the job.

If you don't have a good Ephraim and want a Blade tome buffer, you can just invest in her normally and wait for one with +Spd. She's the best pair up bot for Blarblade users.

I believe +def is one of her best boon if yours has to deal with swords and axes (with TA + Swordbreaker 3)
-Res is the least handicapping bane as she will still resist green mages easily and non blade red mages