Hansel Robles came out of relative anonymity to become a solid bullpen piece for the Mets in 2015. Over 54 innings, Robles struck out more than ten batters per nine innings and posted a 3.67 ERA. He had several big moments over the season, including a strike out of Bryce Harper an inning before Wilmer Flores's walk-off on July 31st. Robles had an impact off the diamond well, reportedly harassing Yoenis Cespedes over text until he resigned with the Mets. He'll start the season suspended for throwing at the Phillies' Cameron Rupp, but his spot in the Met bullpen is secure heading into 2016.
Though he leans heavily on his fastball—throwing it more than 72% of the time—Robles showed some very promising peripherals in 2015. He had the 13th lowest Z-Contact% (contact on pitches in the strike zone) in baseball, better than elite relievers like Dellin Betances and Ken Giles, while also inducing O-Contact (contact outside the strike zone) at a well above average rate. This combination should allow him to hold hitters to a below average BABIP, albeit not as low as the .229 mark he recorded last season. While that number will likely regress, Robles's low strand rate should also improve, potentially balancing any increase in his BABIP.
Things weren't all rosey for Robles of course. He induces very few ground balls—32.8% in 2015, 8th lowest in baseball—and allows a fair number of the fly balls he gives up to leave the yard. Robles's gopheritis was the primary cause behind his elevated 3.91 FIP, and is something he'll have to improve on if he wants to stick at the back end of the Mets bullpen. Robles also allows a fairly high number of hard hit balls, surprising given his contact profile but symptomatic of being a one pitch pitcher with mediocre command.
The signing of Antonio Bastardo likely pushes Robles out of the 7th inning and into a middle relief role, which is ideal for a contending team. Robles is far from a finished product, and pitching in some lower leverage innings should allow him to work on the finer points of his game without costing the Mets late in games. At this point, Dan Warthen could teach a slider to a random fan in the seats and none of us would be surprised, so there's reason to hope that Robles's main secondary offering can improve going forward. Pairing that with his big fastball could give the Mets a useful setup man for years to come.