The Seeker is a title bestowed upon a certain member of the alien race. They, quite obviously, seek information about others. If the old body-snatching movies were allegories for communism, then the Seeker is the big bad KGB gathering information on the god-fearing capitalists and converting them into evil red commies.
The Host blurs all sorts of lines between human and alien, good and evil, so it doesn't really have a concrete antagonist. The Seeker is the closest thing the novel has to a bad guy, claiming the trophy when she shoots poor Wes in the head. The humans take care of that problem when Wanda helps them remove the Seeker and blast her back into space.
With her gone, the body's original consciousness returns, and we're in for a surprise: "The Seeker's host body was named Lacey; a dainty, soft, feminine name. Lacey. [...] Like naming a pit bull Fluffy" (53.1). We also learn that, like Wanda and Melanie, Lacey was there the whole time. This revelation makes the Seeker an even more interesting foil to Wanda/Melanie.
Wanda/Melanie decide to co-exist. Not only that, but they want all humans and aliens to co-exist. The Seeker, on the other hand, is afraid of Lacey. Her strategy for dealing with her fears is to kill them. In that way, her job is perfect. When she sees that Wanda is going through the same problems, she doesn't want to learn from them, she wants to eradicate her.
Unfortunately, removing The Seeker doesn't bring back a princess. Lacey is annoying, to say the least. Wanda says, "Having met Lacey, I'd begun to pity the Seeker" (53.48). We kind of wish they'd loaded Lacey into the rocket right alongside the Seeker. And here's the depressing part: being the first human to be revived means Lacey "was human. She represented hope. She belonged here (53.19).
If this annoying person represents hope, maybe humans aren't worth saving after all.