Connor (Michael Fassbender) she runs into at her house (that's the best way to describe it, since all she uses it for is to eat, sleep, and occasionally lie on the couch), and she doesn't like him, probably because it's a change and/or because she's extremely antisocial. He's just some guy, originally. But then things start to get put into motion (a MacGuffin, perhaps) when they go out to some obscure place where Connor catches a fish and Mia gets bitten. Somehow this really ties them together. They were the only two that went into the water of the four (Mia's mother and sister being the other two).
What ensues afterwards isn't perhaps the most satisfying second half of a film I've seen, but, as a friend says, it definitely beats most stuff out today. Another friend put it as "An Education meets Julia" and that's about right. It might not be the most original climax, but, as even all of the film's detractors have been saying, it's still watchable.
Jarvis is good, and there are not any others that come to mind in place of her. Fassbender, as Nick's Flick Picks said, gives a very good performance, one of those that's subtle. You realize, though, that the movie would not be as good without him. Otherwise, I think people suit their parts well. Andrea Arnold has done a fine job with this movie, a film about watching (as is her "Red Road") and about how if you follow certain obsessions too far, you may not like what you see. A-