Who is Steve from Tomorrow, When the War Began and what is their importance?

Steve is Ellie's former boyfriend. He doesn't physically appear during the story, but he is a huge presence in Ellie's memories. When Lee expresses an interest in Ellie, she puts him off, leaving him to wonder if Ellie is still interested in Steve. Although she admits to having been very involved with Steve, she doesn't seem to be heartbroken over their breakup.