She is played by Lizzy Caplan, as well as various other actors when she shifts into the form of another person.
Tina was born was a soft bone disease, and was prescribed many drugs to treat her condition. She got better due to being exposed to Kryptonite radiation sometime after the 1989 meteor shower; as a result, she gained the ability to change her physical form and appearance, posing as anyone of her choosing, as well as superhuman strength.
In the Season 1 episode "X-Ray," Tina poses as Lex Luthor and goes to the Smallville Bank, where she attempts to clear out Lex's account, only for a bank teller to notice that her signature doesn't match Lex's; this forces Tina to draw a gun and demand that the teller fill her bag with the money. She then flees and runs into Clark Kent, whom she knocks through an eye doctor's window. Clark experiences his first flash of X-ray vision, which allows him to see that Tina has an unusual bone structure.
At home, Tina is confronted by her mother, Rose, about the stolen money, and she defensively insists to her mother that they can have a perfect life, like Lana Lang's, with whom she is obsessed. Tina and Rose then struggle over the bag of money, and Rose ends up falling down the stairs and breaking her neck. Tina starts to call the police, but hangs up.
Tina then plots to kill Lana and take her place, believing she has the perfect life. She finds Lana at the cemetery and seals her inside a coffin in a mausoleum. Clark arrives and battles Tina, eventually stopping her. Tina is sent to a mental hospital.
In the Season 2 episode "Visage," Tina returns to Smallville, posing as Whitney Fordman, Lana's ex-boyfriend who left to join the Marines. To throw off suspicion, she feigns having amnesia, but soon Clark realizes that "Whitney" is actually Tina by using X-ray vision to see her Kryptonite-irradiated skeleton. Aware that Clark is onto her, Tina attains Lana's Kryptonite necklace and, while in the form of Clark's father Jonathan, gets the drop on Clark. She knocks a weakened Clark down into the storm cellar, where she tells him that she realized that she's actually in love with Lana and has also figured out who Lana wants to be with; to that end, she assumes Clark's form. Tina leaves Clark behind in the cellar to die; fortunately for Clark, his spaceship activates and neutralizes Lana's necklace, saving him.
Tina then heads to the Talon, where she finds Lana, professes her love for her, and kisses her. Clark then arrives and convinces Lana that he's the genuine article, causing Lana to resist Tina, who angrily shoves Lana to the floor and turn on Clark, leading to their final confrontation. The fight ends up in the back alley of the Talon, where Tina knocks Clark into another building, only for Clark to hold his own against her. In a fit of rage, Tina lifts up some debris and charges at Clark, only to end up impaled on a pipe. As she dies, Tina returns to her original form and asks Clark to take care of Lana.