Kate Argent

Kate Argent is the secondary antagonist in Teen Wolf season one, she is a werewolf hunter and member of the family that leads them. She is the sister of Chris Argent, and daughter of Gerard Argent. She is also the aunt of Allison Argent, the main female protagonist and Scott's love interest. She has a fantastic hate for werewolves and would gladly kill an innocent one without hesitation and willing to use human's of colateral damage.

Backstory
Six years prior to the series, she seduced an underage Derek Hale, from whom she ubtained secrets from his family. This was likely under her father's orders. Later she leads a group of to burn down the Hale house with Derek's entire family in it. Doing so she knowingly killed a family of innocent werewolves, including children and Derek's human relatives, which broke three very important rules of the hunters code. Sometime afterwards she left with her family.

Return to Beacon Hills
In season one, her brother, Chris, returns to Beacon Hills with his wife Victoria and daughter Allison. She eventually returns when she gets word of an Alpha werewolf killing people in Beacon Hills. She also finds out there are two Betta's, Derek and Scott, who the Alpha is trying to recrut. She tries to get  Allison to learn about werewolves and inforce her world view on her. She eventually abducts Derek and tortures him, eventually using him to reveal that werewolves are real to Allison and tells her that they are all evil monster.

Death
On the night of Beacon Hills High winter formal, Allison finds out Scott is a werewolf. Not willing to put his daughter in danger, Chris tries to have Allison taken out of Beacon Hills by Kate, who has other plans. Kate takes Allison to hunt down Scott. Chris eventually finds out the truth about Kate and her murder of the Hale family, and that Scott is an innocent werewolf. When Scott is cornered at the Hale house, he is wounded. When Allison hesitates, Kate pushes her to the ground and aims to Scott. Chris show's up and threatens her life, revealing her true nature to Allison. At that moment Peter arrives and attacks them, he personally attacks Kate and forces her to apologiese for his family's murder. When she does, it doens't ring true and Peter rips out her throat.

Post Mortem
In season two, she is blamed for the murders the Alpha commited and her involvement in the Hale Arson is revealed. This gained small media coverage of her funreal. Because of this her father arrives in Beacon Hills and declares that all werewolves will be killed to avenge Kate, however it turns out he's using her death as an excuse to cure get bitten by the new alpha, Derek, to cure his cancer.