Well, if you don't like what you have so far (Yahoo Answers' "there is no noun [sic!] with the same root, use 'upright'", or the many suggestions in that WordReference thread — "a good sort", "decent bloke", "a man of integrity", "a man of good character", "principled", "reasonable" and whatnot), then how about this:
Be a mensch.
Merriam-Webster defines mensch thusly:
a person of integrity and honor
Wiktionary even has a few cites:
- 1960, The Apartment:
Doctor Dreyfuss [to C. C. Baxter]: Be a mensch!
- 2005, Alan Hollinghurst, The Line of Beauty, Bloomsbury Publishing, page 428:
Lionel Kessler, relaxing perhaps on a Louis Quinze day bed, garlanded all round with lines of beauty, seeing welcome proof that his clever maligned young friend was a mensch.
- 2008 December 28, George Solomon, “My Little Red Book”, The Washington Post, page D01:
Olie Kolzig: Goalie for the Washington Capitals who spent most of 16 seasons between the pipes for the team until being released in 2008. Had the longest career of any Capital. Now plays for Tampa Bay. The ultimate mensch, in my book.