Blaine Cartier is the sole executor of the Cartier estate. On April 1, 2007, when his sister, Desiree, hosts a party at her mansion for her actress friend, Torrance, she pulled a string of April Fool's pranks on their friends, she pulled one on Blaine by suggesting getting her social nemesis and Ryan's not-so-secret crush, Milan Hastings, tipsy on champagne and seduce her in his bedroom upstairs, however, when Desiree and the others prepared to videotape the affair, Milan has a seizure and falls off the balcony to her death, given the incident as they were in court, Blaine lost control of the family's fortune, which shifts to Desiree, but they are considered innocent and Milan's death is considered a fatal prank.
A year later, living in a apartment, Blaine gets a visit from Desiree saying that they and their friends receive anonymous invitations to gather at Milan's grave at noon, April 1, 2008. Blaine, Desiree, and the others found a box containing a letter about Milan's death and a laptop of Charles drowning but thought it was Charles pranking them until they found them dead. Still, some of their friends are in disbelief, their friends turned up dead one after the other.
Returning to their house, the brother-sister had a brief separation and were later tied to a chair by Torrance who shoots Blaine in the chest which made a chagrined Desiree finally admits it was she who spiked Milan's fatal drink and allowing him to take the blame. However, Blaine, along with their friends and their butler, Wilfred, set this up to make her confess, using props and specials effects Torrance's Boogie Nights 2 set, even the charade involving a revolver shooting blanks, however, as Torrance pulls the trigger, a real bullet caused the top of Desiree's head to be blown off.
Going back to the court room, Torrance was found innocent and since Desiree is dead, Blaine now became the sole inheritor of the family's estate. It turns out, when driving away in Desiree's Mercedes, Blaine had plan this to get back at his sister.
Blaine is the brother of the late Desiree Cartier.
Some time later, Blaine became one of the survivors of the Anti-Hunters.