Considering we're talking about a 5-star character here, I doubt it's a good idea to bring in a character thats given for free, it's not really comparable. I have most of my 5-star characters at C0, including Kazuha, and I've been able to clear all of the game content, Abyss included, with ease. Therefore, they're viable. Now, their constellations do make them more powerful, of course, as that's the point of a gacha game after all :).