[Ryuzaki]

As a user of both ascended youkai right now, I can confirm Byakko is incredibly underwhelming.

Its base youkai form was already pitiful, in comparison to not only Seiryuu, but the other feline youkai, Kilkenny.

Thunder claw's damage, when used by Byakko himself is pitiful, and takes someone building way into 100+ lightning atk to see anything resembling damage if they use it as an evoke skill rather than summoning.
Roar is... Well, minimal damage, tiny AoE, and the actual buff it gives Byakko does NOTHING for him.
There's no DoT, which Seiryuu has, and a beefy one at that.

Overall, for what is the absolute best looking of the youkai, there is currently zero reason to run Byakko other than for theme. Both ascended, and not.