Dinner For Schmucks

Tim Conrad is a mid-level financial executive, who comes up with a way to invite wealthy Swiss businessman Martin Mueller to become a client of his firm. Impressed by Tim's ingenuity, his boss Lance Fender invites him to a "dinner for winners" in which he must find and bring an eccentric person with a special talent; the winner earns a trophy and the executive that brought him or her gets glory. He learns it is more of a "dinner for idiots", and the partygoers will be mocked. Tim's girlfriend Julie lands a curator deal for eccentric artist Kieran Vollard and Tim unsuccessfully proposes to her. After learning of the cruel nature of the dinner party, Julie becomes upset and asks him not to attend and he agrees.
Tim accidentally hits IRS employee Barry Speck with his car when Barry tries to retrieve a dead mouse in the road. Witnessing Barry's bizarre behavior, Tim realizes he is the perfect loser for the dinner. Barry shows up at Tim's apartment and accidentally invites over Darla, who had a one-night stand with Tim years ago and has been stalking him. Barry decides to guard Tim's apartment from Darla to make up for inviting her. When Julie arrives at his apartment, Barry mistakes her for Darla and sends her away, giving Julie the impression that Tim is cheating on her. Barry calls Vollard and finds the number saved in the phone. While talking to him, he and Tim think they hear Julie in the background and head to Vollard's apartment. Barry tosses Tim's keys through Vollard's apartment window and must break in, only to find Kieran preparing to make love to two girls dressed like animals (neither are Julie) and head back home.
Barry opens the door to find Darla waiting outside. Julie calls and asks Tim if he's having an affair and Darla steals the phone and stuffs it down her pants. While Tim hides in his bedroom, Darla and Barry get into a bar-styled fight, scaring Darla away. Julie left her phone at Tim's apartment and a voice message from Vollard reveals that Julie is leaving for his ranch. Barry works for the IRS and offers to take Tim to work to find Vollard's ranch address, where they meet up with Barry's boss and rival, Therman Murch, who stole Barry's wife. Tim leaves the next day for his brunch with the Muellers but Barry crashes, with Darla pretending to be Julie. Tim is pushed into asking Darla to marry him by Barry and Julie walks in during. Tim and Barry chase Julie down to Vollard's ranch, where Tim accidentally offends Barry.
To his surprise, Tim found Barry already at Fender's dinner party. Barry is a hit with the group and is a shoo-in for the trophy, but unexpectedly, Therman arrives, embarrassing Barry with his mind control. Tim then tells Barry everything and, after some encouragement, gets Barry to win with "brain control", before causing his boss and fellow executives to reveal the truth. A guest reacts badly to the news and causes chaos in the process, with Mueller losing a finger. Tim is fired, as is Julie after Barry makes Vollard realize it would be a problem to have her continue working for him. Tim marries Julie, Barry enters into a relationship with Darla, does some artwork with Vollard, and hosts a monthly "breakfast for champions" for all of the losers. Therman writes a book in the mental hospital and Tim gets a new museum started in Switzerland for Mueller. Fender's company goes into a big loss and he is named wall street's biggest loser

Question: Who has been stalking Tim?
Answer: Darla