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Jack is Liz Lemon's boss. He is portrayed as a slick, insincere network executive with an affinity for overtly backhanded compliments, which are usually directed towards Liz Lemon. Although he is clearly the show's antagonist, Jack has occasionally shown a human side.

The first few episodes of season one mainly involved Liz trying to keep Jack from meddling with her show, but, as the season progressed, the character of Jack Donaghy became less villainous and the show became more focused on its characters' personal lives.

(wikipedia.org)