Gage

When Derek's twin brother moves back to Beacon Hills the pack is shocked. They didn't know Derek had a brother much less a twin. Gage immediately takes an interest in Stiles and it starts to cause problems. Derek and Stiles already bicker and argue but now that Gage is home Derek is snappier than usual. Thing can't get any more tense or so Derek thinks. When an unfortunate event puts Stiles in danger Derek blames himself and shuts Stiles out completely, allowing room for Derek's twin brother.