George Kottaras and Theo Epstein have a history. Acquired in 2006 for 5 starts of David Wells, Kottaras spent the first few years of his career as a top catching prospect, with great patience, moderate pop, and the ability to play passable defense. As time took hold of George, he lost most of those talents, and all of his star power with them. What's left is a guy who walks and strikes out with high frequency and does little else. Is that enough to warrant a spot on the 25-man roster?
There are one and a half interesting wrinkles to George's game. The half is his underrated power. The league average ISO for a catcher last year was .143 (oddly enough, that is also the league rate in general). Last year, Kottaras had an ISO of .190, and his career rate is .192. When Kottaras hits the ball, it's generally a long way. The interesting part of his profile, though, is how little he actually does make contact. Kottaras struck out 33.3% of PA last season, and walked 19.0% of the time. That's correct: he didn't make contact 66 times last season, and made contact 60 times (and 5 of those were HR). That's a TTO that would make Adam Dunn blush. On his career (only 820 ML PA), he's the owner of a 13.5% walk rate and a 26.0% strikeout rate. He also averages 24.8 HR per 700 PA. Add it all up, and you might wonder why a guy with a .324 career OBP (and .351/.349 numbers his last two seasons) hasn't had an opportunity to start.
The answer is his batting average. Last season, it was .180. The career slash for the Greek is .214/.324/.406. Honestly, I don't care at all what his batting average is, but it's hard for a manager to see a guy that doesn't get a hit most days he plays (he had a hit in only 18 of his 46 games last year, and never more than one) and want to pencil him in every day. It's perhaps a backwards way of thinking, but it's still there. He also has catcher speed, so his walks mean slightly less than they might with someone who could take 3rd on more singles and so forth. Even with Kottaras' power and OBP skill, he slots firmly at the bottom of a lineup, where there aren't many chances for him to actually come around to score (but that's not exactly his fault).
The plan is almost certainly to start Kottaras against the tougher righties; his career slash against RH starters is .228/.334/.431 compared to a .154/.284/.300 clip on lefties starters. On the other hand, Welington's numbers against righties are essentially identical, so it might just make sense to slot Kottaras in every third day, taking care to avoid lefties, and not worry about "exposing" Castillo against righties.
This is what doesn't make sense to me. Looking at FanGraphs, Kottaras grades pretty highly defensively. He's been consistently worth over a win with the glove. However, if you look at George's CS rates, they are among the worst for the position over the past half a decade. I could steal reliably on George Kottaras. In fact, he is 33% worse than the league average goalie. That's likely a primary driver for Kottaras' consistenly poor FRAA on baseball prospectus. Without having seen him personally, I can't attest to his framing skills or how he calls a game, but the general consensus I've felt is that he's pretty well thought-of with respect to calling a game.
When you combine a poor batting average with an a noodle at backstop, you can make the case that Kottaras should linger in AAA all season. When you combine a 13.5% BB with a .192 ISO, you start to wonder why George doesn't start. Put it all together, and you have the platonic ideal of a backup; someone who is opposite-handed of your starter (Castillo is a righty), who has tools you can play up and serious deficiencies that prevent him from otherwise starting, and a guy who isn't likely to make much more than $1.4 million. I fully expect Kottaras to break camp as the No. 2 C, and play there acceptably all year.