Zack Addy is a major protagonist in the first three seasons of Bones. He is a forensics anthropologist at the Jeffersonian institute who starts the series as a loyal assistant to Temperance Brennan. After the third season Zack stops being a permenent character due to his involvement with the Serial Killer the Gormogon.
In The Series
Zack was probably the "squint" Seeley Booth had the most difficult relationship with as he regarded him as incredibly annoying and was frustrated by him to the point of threatening to shoot him multiple times. Booth was appreciative when Zack got him a toy for his son for Christmas but still only talked to him when he had to. Zack got along great with Brennan though and genuinely respected Booth, helping him get his job back after he was suspended along with everyone else.
Zack became the apprentice to the Gormogon due to his manipulation, helping him perform several crimes. Despite his darker turn Zack still cared about his friends which was proven when he injured himself to protect Jack Hodgins.
A deal was worked out which allowed Zack to stay in a mental ward instead of a prison. Years later Zack was framed for murder but Booth, Brennan and the other squints proved his innocence with the help of Gordon Wyatt.