C'rizz (pronounced Keriss) was a poor bloke from another reality that had to kill his waifu after she got turned into a /d/-tier abomination. He's also the single worst companion that the Prettiest Doctor ever had.
A resident of the Divergent Universe, C'rizz joined the Eighth Doctor and Charlie Pollard after they were exiled there. Some sort of monk with chameleon skin powers, he got shat on over and over as they all traveled the zones, searching for the TARDIS. I'm still listening to the Divergent Universe arc, so I don't know what happens to him yet, but given his luck so far, it can't be good. I feel bad for the guy.
Spoilers: He doesn't get any character development for two years, and as a result, despite having a solid actor, you stop feeling sorry for him as he gets first boring and then annoying and then near-Adric. Right near the end of the arc, he gets two stories where he's an interesting guy. Then there's a big bombshell that explains why he was never a good companion in the first place, but that doesn't retroactively make the last two years of audios that you bought any better, even if you listen to them again. It does manage to make the two where he was good kind of confusing, though. Bravo Russell. He gets better after that, but really, what doesn't get better after the Divergent arc?
I'm also not sure why the CD covers and Big Finish website insist that he's "C'rizz" rather than "Carys". Carys is a perfectly normal Welsh name, just like Terry Nation, even if it's a girl's name, just like Terry Nation. And Doctor Who never ever does stupid apostrophe names, unlike Star Trek and Stargate and Star Wars and all the other F&SF series. And even in bad SF that does use them, they never stand for that sound. But anyway, if the writers are trying to make him seem more alien, they seem to have forgotten that this is an audio series, so it doesn't actually work at all. Besides, Carys (besides being a girl's name) makes perfect sense for someone who speaks English (yes, the aliens in this other universe where the too-un-humanoid aliens got sent by Rassilon all speak English, and their species are named in scientific Greek, and they make it absolutely clear that none of that is the TARDIS translating for us or anything after psyching you out to think otherwise) and appears in a series of stories with a bunch of references to Welsh words and myths.