Genasi

A Watersoul Genasi. Depsite what the growth on the head looks like, this is not an Asari.

Genasi stand on the line between Humans and Elementals. If both of those stand at the far ends on the line, and Archons and Genies are in between them, the Genasi are in between the Humans and the elementals.

Like Tieflings and Aasimar, Genasi owe their existence to Planescape. Like Aasimar, Genasi were basically born of the "grid filling" tendency of the edition, showing up in Planescape bestiaries after the Tieflings were originally introduced in the Planescape corebook. Unlike Aasimar, Genasi were popular enough to get carried over to 4e, whereas Aasimar got their asses dumped for the fallen angel-based Devas. In older editions, there was one Genasi type for each of the Elemental (Earth, Air, Water, Fire), Paraelemental (Magma, Ooze, Smoke, Ice), Negative Quasielemental (Vacuum, Dust, Ash, Salt) and Positive Quasielemental (Lightning, Steam, Mineral, Radiance). Even then, Genasi of the same type could look wildly different, based on what their elemental ancestor - and even if they had the same kind of ancestor, two Gensai could look very different. For example, one Water Genasi might be covered in fish-scales and have webbed hands, a second might look like a humanoid frog, and a third might be a normal human with blue hair.

In 2e and 3e, Genasi weren't particularly popular. 3.5 added a "half-elemental" template that gave some explanation as to where Genasi might have come from, but still, most people chose to ignore them in these editions.

It wasn't until 4th edition that Genasi got promoted to a full-bore playable race, in the Forgotten Realms Player's Guide. This makes sense because FR has always had a fairly substantial population of them; one of the more exotic countries spent a long time under the rule of Genies, so there are lots of genie-blooded genasi running around. Dragon Magazine #350 even introduced the Azerblood and D'hin, genasi of dwarf/azer and halfling/djinn ancestry.

Genasi got a 5e update in the Elemental Evil campaign, appearing in both the Princes of Elemental Evil adventure book and the "Elemental Evil Player's Companion" web-enhancement. Which can be found [if you want to chase it].

In 4e

Genasi are easily recognisable: though of similar built to humans, they tend to be slightly taller and broader in build. That's not their most distinct feature; their skin is of a most definitely non-human color, ranging from brown to blue, purple, red and pretty much anything you can mention. Their skin is also marked with a pattern of lines called szuldar, with the patterns of blood relatives looking somewhat alike. Some amongst them display their lines with pride, wearing clothing that exposes a good amount of skin. Genasi have no hair, though certain manifestations have crystaline growth on their heads.

Caustic, Cinder, Plague and Void Abyssal Genasi.

Every Genasi has an elemental manifestation, which largely determines their character; Firesoul Genasi are passionate in everything they do, whereas Earthsoul Genasi are more contemplative. Pretty much every elemental cliché you know is reflected onto them. Though they are born with only one, some Genasi obtain different manifestations during their lives. Them doing so depends on their environment: a Firesoul Genasi that spends a lot of time with Watersoul Genasi is likely to develop a water manifestation. This tends to happen around adolescence and young adulthood, and though the first change between manifestation can be an uncomfortable, painful and sudden event (nice puberty analogue there, WotC), over time the Genasi comes to control the new manifestation. Nearly all of those who develop an additional manifestation have a clear barrier between the two: though there are some who embrace the strife between their manifestations, and become Elemental Tempests. The most common Genasi are those who share a manifestation with the four classical elements: Earth, Fire, Water and Wind. A common fifth manifestation is the Storm, because everybody likes lightning. A darker aspect of the Genasi is derived from their association with the Elemental Chaos. Because it is tainted with the Abyss, sometimes a Genasi will be born with a corrupt manifestation, and carry the touch of the Abyss with them for the rest of their lives. These are the Caustic, Cinder, Plague and Void souls, twisted versions of the Water, Fire, Earth and Wind manifestations. Lightning doesn't get one.

In Dark Sun they have different manifestations, namely Ember, Magma, Sand and Sun.

The Genasi are a highly emotional race. A desire for change is deep-rooted in the race, to the point that even Lawful Good Genasi embrace the idea of change. Genasi society is small due to the nature of the Elemental Chaos, yet highly dynamic. It is rare to find a Genasi that held the same position for several years, or in some cases, even months. You rise and fall in the eyes of a Genasi based on merit and ambition, but nothing lasts forever, and you'll quickly need to find something to keep proving yourself. In comparison to other races, Genasi settlements know few paupers and less to no permanent poor folk, for a near-impossible ability to feel pessimism and a constant drive to improve one's lot in life quickly restores one's social standing. (as Cruijff would say: "If you're at the bottom there's only one way you can go: up.") The idea of nobility and royalty is foreign to the Genasi, as one's position could be undone at any moment. The downside to this view is that the Genasi don't look to the future, they only know that it'll be different, embracing it whatever it may be.

As is to be expected from their wild and competitive nature, Genasi love competitive sports that focus on the individual, like feats of strength, tests of arms, or in extreme cases, gladiatorial combat. Whereas another master craftsman (for example, a Dwarven smith) might make a weapon that is of great quality, a Genasi craftsman prefers to tailor-make every item to the needs and wants of a customer, a Genasi-made blade is perfect for an individual person. Likewise, Genasi works of art are vibrant, bright items that express the wide range of emotions of the artist.