In case you've read my fiction or seen my art and are sitting here what the h*** I was smoking when I came up with this background, here is the explanation :

The name

I found out there are discussions about how to call Queen Beryl's lieutenants in the non-Japanese fandom. Their canon name is "Shitennou", the Four Kings of Heaven. Which comes in handy to call them as a group, but not as individuals.

One could call each of them a "Tennou", but then that name reminds me too much of the umpteenth manga and anime using that specific part of mythology (for more details on this, check SpiritHime's site about the Manga Shitennou), especially Clamp's RG Veda (very loosely based on the Rig Veda from India).

The name used in the French dub (the one I saw) and the American one is "Generals", which in my opinion makes sense since they have uniforms and seem to lead an army (I have troubles with the concept of 4 Kings and 2 Queens working together O_O )

But none of that was extremely convincing, and given that the "soldiers" that they use are demons called Youma, I decided to call them Shinma (god-demon), a word I picked up in "Vampire Princess Miyu", to describe them as a higher species of demons.

The roots

Actually, "my" version of the Dark Kingdom has been built completely based on the anime, and not at all on the manga.
To be honest, when I later found the manga and read the few said about the Dark Kingdom, I was very disappointed.
In there, the Shitennou had once been the faithful side men of Prince Endymion (Sailor Moon's charming prince) on Earth, a bunch of friends, and had been subdued somehow by evil Queen Beryl in becoming Metallia's head generals.
Meaning, they were not evil, they were all manipulated.
Ew.

When what drew me to Zoi and Kunzite was that they were unarguably villains who liked that, yet somehow had found a twisted way to love each other.

Now that I think of it, they're like ancestors to Spike and Drusilla (... if you don't know where they appear, you're missing something). Their love is as dark as they are, but it's there. And still. Much as I doubt Drusilla loves Spike half as much as he's mad about her, I also had doubts Kunzite loved Zoi, or at least was aware that he did love him.
He treated him more like an affectionate pet whose enthusiastic love is fun to watch, and useful. Which allows less cheesy developments than "Zoi, I loved you at first sight even though our kind is not supposed to know that feeling", or, to stay in a popular manga-canon universe "Zoi, I loved you at first sight even though we're supposed to marry the Moon Senshi".

So I decided to stick to my version and totally ignore the manga.
Anyway, they are barely developped there, and there is no sign that Kunzite and Zoi are as... erm... close as in the anime.

Back to the anime.

Evidence one : stones and plants

The general's names, as well as Beryl's one, are gem names. A majority of youma have a rather plant-like aspect, or sometimes animal-like.

Evidence two : uniforms

Kunzite is the only one who wears a cape. He also wears epaulettes, which everyone including me consider as indicating his rank.
He is, as a matter of fact, the Shitennou's leader.
Nephrite is the only other one who also wears epaulettes - his have a red gem instead of a blue one.
Which makes him second in rank.

There are debates about Zoisite's and Jadeite's respective rank. People usually consider Zoisite as the latest (I think because of both his age and the fact that he seems to be still Kunzite's apprentice somehow. At least they're the only ones we see working together, and in presence of Beryl. There has to be an "official" reason why since Beryl doesn't seem to acknowledge that they have a... erm, bond).

But considering Zoisite's powers, obviously better handled than Jadeite's, I'd rather say that he's equal or third in rank, despite his age. Remember that he does have a mission that seems more important than gathering energy : finding the Ginzuishou.

Evidence three : death

I'll give you that on this point, I stretched it out a little. But hey : I was trying to come up with an interesting background for villains that were barely provided any.

When Beryl kills Jadeite, she traps him into a crystal coffin. His body remains intact.

When Nephrite dies, his body spontaneously dissolves into dust of light.

When Zoisite is struck by Beryl, the Queen orders Kunzite to "get rid of it" (or him?). Which I interpreted as meaning that Kunzite had to destroy Zoi's body after his death, and thus implied that otherwise it would *not* disappear by itself, unlike Nephrite's.

When Kunzite is finally killed by his own weapon, his body too dissolves like Nephrite's had.

Evidence four : blood

I seem to remember that we once spot a cut on Jadeite, and his blood is red.

As is Zoisite's, when Tuxedo Kamen's rose cut his cheek once.

On the other hand, as seen in the previous summary part, Nephrite's blood is green (some think that the green stain on his shirt comes from the plants, but really I don't think so).

Kunzite's blood is never seen, even in the episode when he's hit by his own weapons.

(someone pointed out to me that in the "crossdressing" episode, when Kunzite's hands are hit by Sailor Venus Crescent Beam, you can see red droplets, but given that his gloves remain white, and more importantly that it would ruin my theory ^_^ , let's just say that they were not meant to be blood drops but just a graphic way to express the violence of the shock)

Conclusion

But the previous facts were enough to make me come up with that explanation :
Kunzite and Nephrite are different from Jadeite and Zoisite.
Kunzite and Nephrite were originally born in the Dark Kingdom as Shinma, while Jadeite and Zoi were "made-up" Shinma, which means humans that Metallia had turned into Shinma to replace previously dead Kings.
Based on youmas, I assumed that they, and the original Shinma as well, were hybrid creatures of mineral, plant and animal (in the case of Shinma, humans). Thus the green blood.

The Matter of Love

And so in that version, having no human origin, Shinma don't know love. It's just not in their nature.
They know, from having observed humans for long, what it is, but they just don't feel it. Nor do they feel lust. Because they don't need it. Because they don't reproduce. Because Metallia is like their queen bee, and she, only, provides the seeds that will give youma or Shinma, depending on how they're fed. (and no, she's doesn't need a male, that's not what the Shinma are there for, you perverts ^_^ )

Which explains why it takes quite long for Nephrite to acknowledge that he's getting slightly involved with Naru.

The case of Kunzite and Zoi is different : to me, it all started with Kunzite realizing that he could use Zoi's admiration for him, then letting him believe that his feelings were shared.

And that's where I rather needed a human Zoi in the beginning, as it's doubtful that a Shinma would discover love by himself.

Though I did think of a version where they did. But...

Anyway, there are more details in my fan-fictions on this, since it's at the core of it.
Yup, that is what nourished my version of the Shinma, so don't be surprised if it drifts away quite much from the most common ones...