Fortify Resistance Seal on Eirika?

It works very well even if its only +2, because now that means she can buff e v e r y stat in 1 turn without assistance. Blade users will accept this gladly.

Basically you are having a unit that can buff all stats and if you run a blade user +16 attack in one turn.

She'd be a great support. The moment I got Fortify Res 1 I gave it to her right away. Rally Defense, Sieglinde's hone attack, Hone Speed and Fortify Res will stack a bunch on blade users, otherwise a great support on other units.

Also great on a support Sanaki with rally and hone/fortify skills. Can give up to +18 if you can position with her legendary weapon. +14 if you opt for Rally Attack since it is more reliable.