Is Mark Kiger the only baseball player who has played in the postseason but never the regular season?

Answer 1

He is the guy.
On October 13, 2006, Kiger became the first player since Bug Holliday in 1885 to make his Major League debut in a postseason game when he appeared as a defensive replacement for D'Angelo Jiménez at second base for the Oakland Athletics in Game 3 of the 2006 ALCS. He was added to the A's roster in the ALCS after starting second baseman Mark Ellis was injured in the ALDS.