Kyle is Ian's less attractive, more angry brother. Not a good combination. He's ruled by anger over the loss of his girlfriend, Jodi, who was taken by the aliens about three years ago. Because of this personal loss, Kyle hates Wanda with a passion, blaming her for the cruelties of her race.
He tries to kill Wanda multiple times, and almost succeeds when he assaults her in the bathing cave. It takes a lot of rage for a person to try to and kill someone else with his bare hands. Kyle doesn't seem to realize—or care—that killing Wanda won't bring Jodi back. But it turns out that keeping Wanda alive does bring Jodi back, in a way.
His relationship with Ian is complicated. He alternates between a near-telepathic brotherly love ("without having to speak, they told each other everything" [37.3]), to wanting to kill him and drop him in a ditch. Haven't you ever wanted to beat the stuffing out of a beloved family member? This helps show Wanda the complexity of human relationships and even make us wonder why we act the way we do.
After Kyle learns that the parasites can be removed and their hosts resurrected, Kyle recklessly sets out to bring Jodi back from Seattle. He does, although she's now an alien who calls herself Sunny. Strangely, Kyle is actually attached to Sunny. Wanda observes that she "couldn't believe he'd carried all this gentleness around inside his big angry body" (55.52).
But like all human/alien relationships, this one is complicated. Despite liking Sunny, Kyle wants Jodi back. They remove Sunny, but Jodi's consciousness is too far gone. So they put Sunny back, and Kyle seems happy with it. "He wasn't giving up on Jodi, but as Sunny clung to him, he held her to his side with gentle hands" (Epilogue.18).
Sweet, huh? Well, not really. Is Sunny happy being Kyle's second choice? And what if Jodi's consciousness does return? If we were Sunny, we wouldn't mention it, because that would be like buying a one-way rocket trip to the Dolphin planet.