straight arrow 
North American informal
noun
An honest, morally upright person: he was remembered as a loner and a straight arrow...
- Paul was what we called a straight arrow, maybe the straightest I have ever known.
- Joe was the straightest of the straight arrows, a real gentleman.
- You can tell his character is gonna be a straight arrow when he refuses to let an unctuous lobbyist buy him breakfast.
adjective (straight-arrow)
Honest and morally upright: the straight-arrow head coach found himself answering for their crimes...
- So many Hollywood pretty boys of the past gave us nothing but straight-arrow heroes, while swamping the ageing process in soft-focus photography to ever-decreasing effect.
- Max might like to point out that most of his clients are the tackier celebs, but he's still the uxorious straight-arrow type.
- Hence, when Danielle is coerced back into the world of porn by her edgy former producer, it is down to Kelly's straight-arrow over-achiever to drive to Las Vegas to rescue her, complete with geeky friends in tow.