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 you can use 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.
Edit two years later in reply to comments. No, this word does not have to be spoken to someone with a certain cultural baggage. Here is what Steve Martin tweeted earlier today:
I could not be more stunned by the loss of Robin Williams, mensch, great talent, acting partner, genuine soul.
This went out to 5.2 million followers, not a homogenous bunch by any measure, then got retweeted 50000 times, then picked up by traditional media outlets all over the world.
Mr. Martin does not exactly have a track record of not knowing when to use which word, and we cannot with a straight face claim that everyone who read his message shares the same cultural baggage.
Even more to the point, even if every single recipient did have to look up any word in that message because it was somehow obscure — now it no longer is.