Menelaos' wife and instigator of the entire Trojan war. Although we don't hear about it in the Odyssey, her story would have been common knowledge among Homer's audience.
Here's the deal: Helen was the most beautiful woman ever. Seriously. She was married to a Greek king named Menelaos, which as far as we know was going swimmingly until Paris, the most handsome Trojan guy ever, decided he wanted her. (Actually, he settled a contest between three catty Goddesses and "won" her, but that's another story). So he stole her. The Greeks launched a war against Troy, with many ships, swords, spears, arrows, and deaths, and they won: Menelaos got his wife back.
In the Odyssey, we meet Helen when Telemachos visits Sparta for news of his father. She's very gracious, telling him stories of Odysseus and managing not to start any wars throughout the entire dinner conversation. (This might be helped by the fact that she drugs everyone's drinks.) At the end, she gives Telemachos a dress for his future wife to wear at the "lovely occasion of your marriage" (15.127). Until then, she says, his mom can hang on to it. Because that's not creepy at all.
For more on this gal, check out Shmoop's guide to Helen.