Jack the Grunt was an alien grunt from Xen and the son of Lyn the Grunt.
Jack and Lyn were involved in a conspiracy with Tony Angelo to cripple Nathan Johnson's counter terrorism operation which was investigation the ZOMBIE Facility in City 19. Jack was sent to spy on Nathan.
Jack kidnapped Emile Clayton and brought her to Tony Angelo to use against Nathan.
In his true form, Jack looked like any other grunt type, except was smaller in size. In his human disguise, he had the appearance of a chubby child and had blonde hair and brown eyes and swollen cheeks. This appearance was caused by his true form underneath his human suit.
Behind the Scenes
Much of the character's fate is left uncertain, as the story arc concerning him was cut short. In the City 17 Street 2013 book some notes are added giving some possible insight. It is shown that he had Emile kidnapped (which didn't occur in the actual show) and was later murdered along with his mother Lyn and father by Frank Blackstone. But as this wasn't officially stated in the actual series, the canon is unclear.