I think I know what you are trying to say. I'll respond asuuming you're trying to say that yuna is useless in a fight.

Yuna is far from useless. She can heal and summon for straters, both of which are necessary unless you have extrtemely good characters. Secondly, once she has completed her "path" her magic is almost double of Lulu's. So if you get Lulu to learn all the firagas, thundagas etc. and then transfer them to Yuna via. black magic spheres (or numerous other ways) she then becomes infinite times better than Lulu. Her path also goes by Rikkus "steal" and "use" commonds, so if you if you retern with a return sphere you can learn those as well. Add moves like "doublecast" and "holy" into the mix and she becomes by far the best character. If you play it right she will be able to summon, use wht magic, black magic, steal, use special items and have excellent stats. This is all withut going out of your way to do anything because pretty much all the required items can be obtained from bosses or findable chests.