It's been stated earlier, but basically her buffing ability is very good, mixing good generic attack and max brv buffs with a potentially powerful aura, can debuff enemy max Brave by half and reduce their attack a little, all her attacks restore Brv to the party, especially her second one, and at the center of it all, she has two distinct poison effects, one from her First ability and one from her EX.
She's a great all-rounder that specializes in party stat support and denying the enemy it's Brv, while supplying her party with Brv of their own.
She's pretty good, but you want to get at least three of her Dreaded Artifact Passives, as they inform much of her strength, since without at least some of them, her Max Brv is absolutely unacceptable.
To put it into perspective, she is on par with other characters and their buff passives to max Brave, when she has all her Artifacts, giving her a total of 90% Max Brv. without them, she will basically be at half max Brave compared to the rest of your party.
So in conclusion, she's a fine addition, but I'd honestly not worry about it if you only want to use her for Shiva. Do a quick half hour Eidoja grind with her World of Illusion, grind when Zack or Vayne's show up, or just don't worry about it if you have any other useful supports like Lenna. She doesn't need her EX to be useful, but she'll lose out on functionality if she can't get it.
