To the momentary alarm of his parents, Zachary Goodweather was born wholly encased in a caul. Hence, as described in 'The Fall' novel, at the event of his birth he "was not yet born", continuing to feed off his mother as he were still inside her womb. During the opening sequence of the Season 3 episode, "First Born", his mother Kelly interprets his delivery from her womb while still being attached to her as a sign that fate has forged an eternal unbreakable bond, that can never be torn asunder. He grew up to be asthmatic and carries around an inhaler.
Just like any other kid his age, he plays video games and is smarter than given credit. Smart enough to know that his father's job is bigger than him and if he doesn't let him work (even though his dad would rather have custody) they may both end up dead, along with the rest of the world.
After his mother is turned, he has to remain on the run from her (to be expected) "Dear One" obsession. Eventually, he is left alone in the subway tunnels after having to split up from Nora because her mother is causing too much of a disturbance. Nora has him hide but he can't hide forever as the Master minion's finally find him and take him in.
Unexpectedly, The Master doesn't turn him but keeps him as human to learn more about his father Eph, as well as other nefarious purposes such as grooming him as the next host. His role was pivotal in the gambits between two father figures, the Master and Eph, in their ultimate confrontation. By means of a major psychological twist, Eph called the Master's bluff and recognized they all had to die for his son to be released, otherwise the Master would prevail. At the end of the book series, Mr. Quinlan, Eph, and Zach are killed along with the Master and his strain when a nuclear bomb is set off in Thousand Islands, Ontario.
Zach's role appears the same from the novel. He's smart enough to get the grasps of the reality that's going on as the Master released the strain that turn many people into vampires. Zach struggles with Eph about knowing the truth. While others like Dutch and Abraham Setrakian don't mind telling him the reality, this angers Eph. His father wants desperately to shield him from the truth and procrastinates over telling him of Kelly's most likely fate. Though despite this, Zach seems interested in fighting and even killing vampires like the others. When season 1 ends, Zach sees his mother but refuses to believe that she's turned due to her calling his name.
When season two begins, Zach is obviously still angry. Ephraim tries to explain that Kelly is no longer herself and that she's "sick". Zach angrily states that his father is a doctor and he had to save her. He also emphatically demands his father to find a cure. Ephraim eventually gives in and tries to figure a cure, but it is not a simple process as it sounds. Due to the "strain" being foreign to modern science, he and Nora are diligently trying to resolve the matter.
Zach becomes even more angrier when Ephraim will not tell him what he's doing and he won't explain how he's trying to figure a cure. His anger becomes more evident when he looks at the board where they write out their conclusions. He angrily tries to rub out "infected", hinting he does not want to believe Kelly's lost.
Zach is abducted by his fully matured strigoi mother after she bit Nora who later commited suicide. Kelly did not turn her son but took him away with her. Her motivations concerning her son have yet to be revealed. She did not succumb to the desire to turn him immediately but she is a sentient strigoi with a degree of self-control.
Zach spends the majority of the third season, hidden underground in Central Park with his mother and Thomas Eichorst. After witnessing his mother feeding on a young child, he attempts to escape only to succumb to an asthma attack, in which The Master uses his white to cure. Later, Zach is given a Feeler to protect him, and later goes out at night with. He and Kelly move to Eldritch Palmer's penthouse after The Master elects Palmer to be his new host body in which Kelly is entrusted with a detonator for a nuclear bomb, by Eichorst. After The Master is temporarily subdued inside a silver box, Kelly attempts to infect Zach, after no longer hearing his voice. Zach tries to take his mother to see The Master (believing him to just be upstairs), only to find his father, Eph, recovering from injuries sustained moments prior at the hands of The Master. A fight breaks out between Eph and Kelly, which Zach hopelessly tries to stop, resulting in Kelly's death. Out of anger, Zach sets off the nuclear bomb, and after finding Eichorst, leaves with him, after Eichorst commends him for what he's done.
In the fourth and final season, Zach is being cared for by The Master himself, residing with him in the Empire State Building, who takes an unsettling interest in the boy. The Master now in Palmer's body administers him the White but keeps the boy human. It is likely through the white and his immense telepathic ablities that the master can influence Zach slowly. Zach is not mature enough to see through the machinations of the master as he hits puberty and is therefore vulnerable to anything that strokes his ego. The Master continues to nurture Zach's love for his deceased mother further driving a wedge between him and his father Ephraim. This gentle side of the master irks Eichhorst mercilessly who voices his dissappintment in Zach to the master almost to a point where the master doubts Eichhorst's usually unwavering loyalty.
Zach's dark side surfaces more when he befriends his maid, teenage girl Abby, whom he eventually develops romantic feelings for. However, after learning she has a boyfriend and believes her to have been using him, he allows one of his Feelers to turn her.
After The Master learns that Quinlan and Fet have brought a nuclear weapon back to New York, he sends Zach in as a spy back to his father's hideout with the intention of locating the nuke. However, Eph soon sees through his son, pointing a gun at Zach's head, but could not bring himself to do it and Zach ultimately does not learn of the nuke's whereabouts.
In the series finale, Zach remains by The Master's side as the group launch their last ditch attempt on The Master. Zach accompanies The Master and his Strigoi horde for a final showdown, and after The Master kills Quinlan, he orders Zach to kill his father which he cannot bring himself to do, instead shooting The Master from point blank range. The Master, whose host body was wounded during the fight with Quinlan, attempts to transfer his essence into Zach, but he is saved at the last minute by his father, who in turn, The Master transfers his essence into instead. After reconciling his father, Zach hugs his body, which The Master has taken over and hoping that apart of his father still alive within, say's 'I love you' to his father before setting off the nuke and killing himself and The Master.
- A caul is fetal membrane from the amniotic sac encasing a portion of a baby at birth. It is a rare occurrence for births that have come to term but is more common, though still very exceptional, among premature babies, which tend to be wholly encased in the amniotic sac. Not unlike umbilical cords, cauls are valued as mementos and charms.
- In superstition, a caul is commonly regarded as an omen of auspiciousness or otherworldliness, with the caulbearer (i.e., the caul baby) destined for greatness, as in the case of Julius Caesar, or having connection with the spirit realm, as in the case of benandanti. In a darker East European account, folklorist Agnes Murgoci relates in 'The Vampire in Roumania' (1926) that caulbearers cast the evil eye and become vampires after death, especially if some of the caul is swallowed during birth.
- Benandanti (lit. means "Good Walkers") was a fertility cult found in Friuli, or North East Italy. They were known for their ability to commune with the dead, battle witches, ensure a bountiful harvest, as well as a preternatural skill to heal the sick and injured. These peasant seers were often sought out by local villagers in matters of the occult and persecuted by the Inquisition during the Middle Ages.
- Numerous counts of New York City citizens and survivors (Caused and Direct)
As Zach detonated the nuclear device. The nuclear radiation will created a nuclear fallout that reached New York killing anyone outside and also letting millions of vampires to wander around the streets of New York.