Okay, Tony Pena was not an "unknown" per se. However, he was past his prime by the time he got to Boston and he certainly was not one of the better hitters on the team. Pena was overshadowed by Wade Boggs, Mike Greenwell, Ellis Burks, and others. But Pena's real value was his defense behind the plate.
Pena was a fun player to watch, with his unusual catching stance which involved him stretching one leg out to his side. Despite this, he was a strong thrower, throwing out between 32 and 37% of base stealers in his four years with Boston. Pena won the Gold Glove in 1991 with the Red Sox, the last Red Sox player to do so until fellow catcher Jason Varitek in 2005. That year, he lead the league in putouts and runners caught.
Unfortunately, his bat was never as impressive as his glove, particularly in 1993, his final year when he put up an embarrassing .502 OPS. He certainly was not the same player as he was with Pittsburgh and St. Louis when he was an All Star several times and widely considered one of the best hitting catchers in the league. But Boston did not bring him to the team for his offense. It was his defense they were after and Pena certainly delivered.