To be fair, I really didn't know who he was either. I think I recall John McLaren mentioning him during spring, but accidentally calling him Juan Castro. Anyway, he'll be taking over the hitting coach duties as Lee Elia goes to bench coach. Elia will supposedly still oversee Castro.
One quick question.... DOES IT REALLY MATTER? At this point, the Mariners don't need a hitting coach, they need an exorcist, a couple live chickens and a few one-way plane tickets for some players. Look at the numbers -- thanks to baseball-reference.com
Anyway here's Mr. Castro's bio provided by the Ms.