An Offer You Can't Refuse: When Snape catches "Evan" raiding his potion stores, "Evan" forces him to let him go without punishment or telling anyone else or a flashing neon sign reading 'Dumbledore Took My Virginity And I Liked It!' will appear on the back of Snape's trousers.
Death Glare: "Evan" keeps glaring at Harry through potions class. This leads Hermione and Harry to conclude that the new kid has a crush.
Didn't Think This Through: in a state of panic, "Evan"/Voldemort creates massive wards on an abandoned classroom door that only Dumbledore or Grindelwald could hope to break in the hopes of avoiding further interactions with Harry and Draco. He realizes the gaping flaw in his plan when he begins to worry that McGonagall might get around his wards if she were to transfigure the door into something else and as it is Harry circumvents the door and all of his carefully laid warding altogether by dropping in through the window on his broomstick. And because he's so heavily warded the door he's essentially locked himself in.
Laughing Mad: Voldie when he realizes that Potter won this round by snogging him.
Let's Fight Like Gentlemen: Voldemort wants to beat Harry fair and square so that no one will ever say he was bested by a child again and so waits for Harry to draw his wand so they might duel properly.
Mix-and-Match Man: for his plan Voldie developed the "Evan Farley" identity by using a potion similar to polyjuice but instead of just copying the appearance of one person he creates a new identity by using several different donors.
Sexual Karma: As Draco oh-so-helpfully points out "Evan" is the "girl" of the relationship.
Surrounded by Idiots: Voldemort wonders how hard is it to kill a teenaged boy? After several failures by his pathetic underlings Voldemort decides that "if you want something done right, do it yourself" and hatches his plan to infiltrate Hogwarts as a student.
Time for Plan B: After the initial fiasco Draco comes up with Plan B—that Evan should "seduce Harry and kill him while he's distracted." Unfortunately for them they don't work out the details of how Evan is supposed to do that when he himself is distracted.