Derek first appears in "Catch-22" in Desmond`s flashback.



Derek is first seen entering the monastery where Desmond works, while Desmond is packaging wine. He walks right up to Desmond and punches him in the face. He then apologises to Brother Campbell before leaving. Desmond later visits Derek's house, and it is discovered that he is the brother of Ruth, Desmond's jilted former fiancee - this is presumably why he punched Desmond in the monastery. He is reluctant to allow Desmond in, but is interrupted by Ruth, who lets Desmond in.

Unanswered questions about Derek:
How did he know Desmond had joined the monastery?

I wonder if it was the monk or mrs.hawkin who told Derek where to find Desmond, in order to push him towards his future with the button-pushing?

Didn't Ruth know he joined the monastery or did she only know because Derek knew?

Desmond couldn't really tell anyone because he was under a vow of silence. But you'd think the monastary as a responsible "employer" would make his where abouts known to at least Desmond's brothers. The family would be worried if Des just disappeared and then there'd be coppers poking around the joint looking at the time machine hidden away behind the wine casks, wouldn't be good.  

That`s true, the monks might have told his family and they in turn told Derek/Ruth.