Technically not one of The 4400 returnees (she was still in the womb at the time), Isabelle Tyler (Megalyn Echikunwoke) is the daughter of Richard and Lily Tyler. Even while in the womb Isabelle proved to be very powerful, and as an infant she used her abilities to protect but also manipulate her parents. It was her blood alone that was able to save the 4400 when their lives were threatened by the promicin inhibitor.
Isabelle instantaneously jumped from being a toddler to a young woman, a dramatic transformation that also caused her mother Lily to age into an old woman. But Isabelle was not prepared to take her own life, thereby facilitating Lily's death. Isabelle struggled with her rapid maturation, turning to Shawn Farrell for guidance and eventually become romantically involved with him. She was torn between her feelings for Shawn and her destiny to destroy the 4400, which according to Matthew Ross was the reason she was given limitless abilities.
She began to assert her own will, killing Matthew and executing Nova Group members who'd attacked Shawn. Declaring that the fate of humanity was at stake, she convinced Shawn to marry her. The wedding was called off, however, thanks to the surprise reappearance of Jordan Collier. When Collier ordered her out of the 4400 Center, Isabelle turned to Dennis Ryland, allowing his researchers to extract her promicin in order to develop a supersoldier army.
When the promicin she donated was stolen, Isabelle tortured Shawn for information about its whereabouts. When she arrived at the location with Ryland and the supersoldiers, they walked right into a trap. Isabelle managed to escape death and went on a murderous spree at the 4400 Center in an attempt to draw Collier out. Instead, she was confronted by NTAC agents Tom Baldwin and Diana Skouris. But it was her father Richard who ultimately ended the rampage, injecting Isabelle with a syringe that took away her abilities. The confrontation left her injured, powerless, and in NTAC custody.
The "evil" Tom takes Isabelle to a secluded beach house and convinces her to fulfill her original mission of destroying the 4400. First, he rids her of the anti-promicin allergens that the government gave her. Then, he sends her back into Promise City with the warning that if she tries anything funny, he can activate the newly installed "kill switch" and she will die. When she gets back to Promise City, she is greeted by an ebullient Kyle.
Now two-years old, Isabelle remembers nothing about her former life. However, Richard relents and brings Isabelle back to her former age when a virtual Lily appears and makes him feel guilty.
Soon after, Tom and Diana find where Richard and Isabelle are hiding out. The now marked Tom tranquilizes Isabelle and brings her back to his house. He tells her that she needs to complete her old mission: stop Jordan Collier.
Isabelle awakes to find herself trapped at NTAC with Kyle, Collier and many others. Eventually, PJ confesses that they are all trapped because he took promicin and developed the ability to draw others into a game that takes place in a shared psychic space. In the game, the players have to work together to overcome an obstacle; in this case, the obstacle is NTAC itself. When the building kills off Meghan, Shawn, and PJ, Collier and Tom join forces, however briefly, and save everyone by short-circuiting the building.
Cassie directs Kyle and Isabelle to a secluded area where they find Collier bathing. Collier is shocked to see the young man who shot him and the woman who wanted him dead.
Collier asks Isabelle why he should trust her. She tells him that she's not in the White Light book and no longer has any abilities. However, Kyle has assured her that the movement needs her there. Collier lets her in, but remains skeptical.
As the search for Isabelle intensifies, Kyle and Isabelle take refuge at a bunk house at Camp Chinook. Isabelle is still unsure of how Kyle intends for her to help the movement, but Kyle tells her to be patient.
Meanwhile, Isabelle offers to help Kyle translate the coded portion of the White Light text. Eventually she succeeds and is stunned to find out that the coded portion lists the names of people who need to take promicin to ensure the movement's success. The final name on the list is Tom Baldwin.
At a government detention facility, government bureaucrat Jack Delphy tells Isabelle she may get released from prison if she gets tested for her tolerance for Promicin. Isabelle agrees to the test, and the results confirm that Isabelle has an allergy to Promicin. If she takes the shot, she will die.
While in jail, Isabelle is visited by Tom. Isabelle suggests that the future took Alana away from Tom because he didn't kill her—Isabelle—as he was instructed. Isabelle in turn asks Tom if he knows anything about Shawn or her father Richard. When Tom presses her for more details about Alana, Isabelle tells Tom "you don't wanna know what I know."
SEASONS ONE-THREE TIMELINE
Even from the womb, it is clear Isabelle has extraordinary abilities. Already she has the ability to communicate with her mother as well as exercise extreme power over things around her.
In an effort to protect her parents, unborn Isabelle unleashes a paralyzing pain on Jordan Collier when he touches Lily's pregnant belly. As a baby her powers continue to heighten. While in hiding and harassed by religious zealots, Isabelle saves her parents by willing death upon their stalkers.
Maia Skouris accurately predicts Collier's assassination but baby Isabelle "shows" him another version of his future to intentionally mislead Collier, giving him a false sense of security.
With The 4400s in quarantine, Isabelle's untainted blood is the only thing capable of curing Shawn, who in turn, heals the rest of The 4400s.
When Lily next checks in on Isabelle, she finds her daughter missing, not knowing that miraculously Isabelle has grown into an adult woman.
Isabelle is startled and confused to awaken one morning no longer a baby but a grown woman. She soon meets her mother who has also aged overnight.
Isabelle tries to understand her new life and educate herself, seeking comfort and knowledge from Shawn. The pair begin to form a new friendship.
Realizing she is responsible for her mother's decreasing health, a conflicted Isabelle attempts to kill herself by jumping off the 4400 Center roof. Amazingly, she walks away unharmed.
Following her suicide attempt, Matthew informs Isabelle of the futures "plans" for her. There is only one thing that can kill Isabelle: a substance Matthew gives her. Isabelle now needs to make a choice between her life and her mothers. Ultimately she chooses her own.
Isabelle and Shawn grow close and she asks him to "be her first." Regardless of her father's advice against the relationship, and Matthew pushing her towards it, Isabelle kisses him and he is unable to resist.
Shawn tries to end his relationship with Isabelle, but she will not allow it. Convinced that her father is causing of Shawn’s doubts, she uses her abilities to momentarily harm her father.
Convinced the NOVA group is responsible for Shawn's behavior, Isabelle believes the only way to save Shawn is to hunt down NOVA herself. When Matthew tries to stop her, Isabelle kills him, telling him it is time for her to make her own choices. Isabelle proceeds to take down the NOVA group, one-by-one, using her abilities. Later, Richard convinces Isabelle to turn herself in but both he and Tom wonder if she is innocent or a cold-blooded murderer.
When Shawn's ex girlfriend shows up, Isabelle gives Shawn a subtle warning to stay away from her.
Isabelle tells Shawn that they should get married. She insists this is the only way that Shawn can help stop her from doing the horrible things Matthew predicted.
Worried about Isabelle’s capacity for evil, Shawn tries to kill her while she sleeps. When he does not succeed, Isabelle explains that Shawn doesn't have a choice. Marriage is the only answer.
Isabelle begins making preparations for her wedding, preparing the guest list and asking her father to plan his speech.
Isabelle learns from her father that the wedding has been postponed because Jordan Collier was seen at The 4400 Center.
Worried about Collier's return, Isabelle reveals to her father that she showed Collier a false vision of the future when she was just a baby. Richard notices that Collier seems to be the first person Isabelle has ever been afraid of.
When Jordan finally returns to the Center, he finds Isabelle waiting for him. Although Collier forgives her for what she did in the past, he tells her to leave the Center because she is the enemy of the 4400 and will never be able to change.
After leaving the 4400 Center, Isabelle joins forces with Dennis Ryland and promises to offer him protection from Collier and the 4400. Together they continue to build an army with 4400 abilities.
Isabelle continues to work with Ryland preparing for a confrontation with Collier. When Shawn is arrested by NTAC, Isabelle and Ryland show up in an attempt to interrogate him. Although Isabelle admits to Shawn that she once loved him, she spares no remorse and uses her ability to force information about Collier and the promicin supply out of her former fiancée. But when Isabelle arrives to destroy Collier she finds he's fled.
Resolving to bring Collier down once and for all, Isabelle visits The 4400 Center, determined to kill off as many people as it takes for Collier to show. When Tom and Diana arrive, a struggle ensues, but is cut short when Richard injects Isabelle with Tom's white syringe. But instead if killing her, the liquid takes away her abilities and Tom, in an effort to makes sure she no longer has powers, fires a round into her arm. A wounded Isabelle is taken to NTAC Medical.