What do you call someone who's very stable all the time? By that I mean someone who never lies, manipulates or cheats. Someone who doesn't understand jokes sometimes, and someone who never gets overly excited. But not someone who's boring. He just has a calm atmosphere. There may not be a word for it, but I want to know how I could describe someone like that.
I've always used the term "straight arrow" to refer to such a person, especially if I wanted to emphasize the "never lies, manipulates or cheats" aspect. The "doesn't understand jokes sometimes, and never gets overly excited" part follows (to some degree) from always acting in a responsible and upright manner.
If you're looking for a word for someone who is not shaken by external circumstances, then equanimous is the word for you, but it might not necessarily imply (at least not to everyone) that they never lie or cheat etc. In that case saint would probably be better.
From the blurb beginning the Wikipedia article:
Equanimity (Latin: æquanimitas having an even mind; aequus even animus mind/soul) is a state of psychological stability and composure which is undisturbed by experience of or exposure to emotions, pain, or other phenomena that may cause others to lose the balance of their mind.
If they are calm under stress, I would use the term "unflappable".
If they also don't get excited when they should be happy, though (such as during a party, or sex, or the birth of a child), you could try "composed", "cool", "low-key", "wooden", or "stoic".
I don't think that there's any connection between honesty and stability. Also, you described the person as not boring - such a person may not exist. If they're playing it cool when they should be excited, they probably have what psychologists call "flat affect", which is not a positive quality.
The Oxford dictionaries define "stolid" to mean "calm, dependable, and showing little emotion or animation". "Phlegmatic" is another possibility with a definition of "having an unemotional and stolidly calm disposition". "Solid" and "sound" give the same sense of reliability without so much emphasizing the lack of affect.
Reliable, stable, dependable, trustworthy, passive in a sense, maybe little naïve, or has a child-like innocence, honest, wholesome. Maybe he is a bit idealistic, because you said he was fair and honest. These are all good ways to discribe the person or character's personality. He kinda reminds me a Boy Next Door type of guy. Hope this helps.