straight as an arrow
Honest, genuine, as in You can trust Pat with the money; he's straight as an arrow. This simile alludes to the arrow's undeviating flight through the air. [Second half of 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for straight as an arrow
Or is he just a straight-as-an-arrow Eagle Scout, far more Clark Kent than Superman?Calvin Coolidge’s Bone-Dry Humor Is a Lesson in Laughs for Candidates
February 20, 2012