Daniel is Julie's husband. Yep, that's right – husband. He's the same age as Julie, and his mother promises that he'll be just like a brother to our young heroine. Unfortunately, this prediction doesn't come true.
Early in their marriage, Daniel keeps to himself. We learn that he "has a few problems" (2.76), and when Julie first meets him, "she knew from his grin and dull eyes that something was wrong with him" (2.75). It appears Daniel is developmentally challenged.
Unfortunately, this makes him the target of teasing from the other kids in town, who make fun of him saying, "Dumb Daniel. He's got a wife and he can't mate her. Ha." (2.119) While that's clearly awful, it definitely doesn't excuse what he does next.
Prompted by the neighborhood teasing, Daniel attacks Julie in what some describe as an attempted rape. It's clear, though, that Daniel is suffering, and our narrator tells us, "He was as frightened as she" (2.122). But it's worth noting that the threat continues; when Daniel runs off, he whimpers, "Tomorrow, tomorrow I can, I can, can, can, ha ha." (2.123)
This incident and Daniel in general totally terrify Julie. His attack (along with the threat of another just like it in the near future) prompts her to attempt to run away from home: we have Daniel's sad, misguided aggression to thank for Miyax's current predicament.