Tom Marvolo Riddle (Lord Voldemort, if you're nasty) has been with us since the very beginning of the Harry Potter film series, but he spends most of his time laying low in Order of the Phoenix.
While Voldemort is alive and well (even if he does look like a weird, pale snake person), he's not broadcasting his return to the wizarding world just yet. He's spending his time building up his army. He's trying to recruit giants to join his side. He breaks his loyal Death Eaters out of Azkaban. Dumbledore also suspects that Voldemort is behind the dementor attacks in Little Whinging. You know dementors can't wait to start sucking out souls when the Dark Lord takes over.
Lucky for Voldemort, he's getting a lot of help keeping his return under wraps. Harry and Dumbledore are trying to blow the whistle on him, but He Who Must Not Be Named strikes fear into the hearts of so many that the higher ups wizarding world would rather just pretend all is well than admit Voldemort might be back.
That's how scary this guy is.
When Voldemort figures out the connection between him and Harry, he uses that to his advantage as well. Voldemort shows Harry a vision of Sirius being tortured and Harry rushes off to save his godfather. For Voldemort this whole loving-other-people thing is a major weakness. (We bet he hates hugs, too.)
That's why, when Voldemort tries to possess Harry, he shows him all the worst moments of his life. Voldemort thinks this is a pretty great plan, but Harry has a secret weapon:
VOLDEMORT: So weak. So vulnerable. Look at me.
DUMBLEDORE: Harry, it isn't how you are alike. It's how you are not.
HARRY POTTER: You're the weak one and you'll never know love or friendship. And I feel sorry for you.
Yes, Harry's secret power is love and friendship. Voldemort can't stand to possess Harry when he's got all these warm fluffy feelings inside his heart. Voldemort is a cold-hearted, broken soul and it's awful being so close to someone who actually likes people.
Awww. We kind of feel sorry for Voldemort now, too. Kind of.