Jack Harper is a minor character in the books, Tiger and Beast Squad.
Jack is the Freelancer Foundation's helicopter pilot and he is also Tiger's rival and coincidentally, also his next door neighbour. While Tiger remains alone most of his life, Jack proved quite the ladiesman, as he sleeps with a girl at least once a week. There has been speculation that Jack slept with Sarah Damen, with whom Tiger fell in love with. When Tiger found out, he was deeply hurt, but his feelings towards her remained.
When Tiger was in a coma, Sarah realized that Jack was only using her for sex and rebuffed his advances since, stating that 'there is a better guy out there' refering to Tiger, who saved her life when Bobby Salvatore took her hostage and put Tiger in a coma in the process.