Carter, 54, was a 2-time World Champion (1992-93) with Toronto, and in the latter World Series, famously hit a walk-off, three-run home run off Mitch Williams to give the Blue Jays their second straight title. He was also a 5-time All-Star and 2-time Silver Slugger over the course of his 16-year Major League career. He had 12 seasons with 25-plus home runs and 10 campaigns with 100-plus RBI, highlighted by an AL-best 121 RBI in 1986 as well as becoming the first player to record at least 100 RBI with three different teams in consecutive seasons (Indians, Padres and Blue Jays from 1989-1991).
Over the course of his career, he also played for the Cubs, Orioles and Giants, and In a total of 2,189 career games, Carter hit .259 with 432 doubles, 53 triples, 396 home runs and 1,445 RBI. After retiring at the end of the 1998 season, he spent two seasons as a color commentator for the Blue Jays, and two more in the same role with the Cubs. He was a "one and done" in terms of the baseball Hall of Fame, getting 19 votes (3.8%) during his first year of eligibility in 2003, and being dropped from future ballots. However, he was inducted into the Canadian Baseball Hall of Fame the same season.
It seems a little bit of an odd choice, since unlike other team "special assistants", e.g. Randy Johnson, Luis Gonzalez, J.J. Putz, there's no obvious Diamondbacks connection here. There is an obvious connection to Stewart, alongside whom Carter played for Toronto in 1993 and 1994, but it's not as if the team will be retiring Carter's number anytime soon. One wonders what his role might be: mentoring young players? I'm pretty much speculating here. But as Jeff Summers put it, "If they ever have a front office softball game, they're stacked."