Iku does +72% att/mag, but the chance of paralyze, the single target, and her low physical durability makes it tough to use.
Iku is also one of, if not the, BEST magic tanker in the game. And magic attacks are FAR more common on bosses then physicals; a vast amount of bosses have one or no physical attacks at all, including Yukari and the
final boss (after you take out the Shredding Amneseri or whatever that addon is).
Having a paralysis-immune party is a pretty good thing overall anyway. And the damage boost from Iku FAR surpasses what non-par-resistant equipment would bring.
The single target doesn't really make it hard to use either, as you'll probably only have one or two people out at a time who'd need the buff anyway, and due to the big amount of buffing it gives, it doesn't need to be renewed often on non-superfast-attackers.
About Rinnosuke, it's worth mentioning that Shining Stars of Traumeiri or however you spell it does a decent amount more damage then Scarlet Gold Sword when you've got buffs on him, assuming neutral MYS affinity and non-major defensive stats. But in general, Rinnosuke isn't really a very exciting character. All his stats are above average, but none of them are particularly good either. He's a lot like a Remi with worse HP/Speed/Delay and no self-buff in exchange for better skill variety.
