Dalar

The dalar (both singular and plural are "dalar") are a humanoid but nonhuman species living secretly alongside humans on modern-day Earth. The dalar are actually quite close to humans, either another subspecies of homo sapiens (homo sapiens dalaris, perhaps?) or another species in the same genus (i.e. homo dalaris). Regardless of which they are, they appear physically quite similar to humans and the two species are genetically compatible enough to share (most) diseases and have children together.

Biology
The dalar are physically very similar in appearance to humans, both externally and internally. However, as a result of their exposure to and use of amati, they live to be around 500 years old (barring deaths by non-natural causes). Due to their small population, there's a lot less genetic diversity than among humans, leading to less variation in appearance. As a result, the majority of dalar are, on average, somewhat taller than humans (average for both men and women is around six feet), with brown eyes, dark brown or black hair, and "medium" brown skin. Their proportions, especially in their face, is subtly off from that of humans. To someone who's familiar with the dalar, it's not too hard to guess that someone's dalar.

In terms of aging and maturity, dalar age much more slowly than humans do. A dalar 10-year-old is about equivalent to a human 6-year-old; a dalar 30-year-old is like a human middle schooler; and a 48-year-old dalar (the age of adulthood) is equivalent to a human who's 16-18. Dalar are basically physically mature in their 40s, but will continue to grow slowly in height for another couple decades. (Most dalar are between 5' and 5'6" when they're in their 40s, and reach their full height of around 6 feet closer to 100.) They are considered full adults at around 100 (kind of like how in the US, you're a legal adult at 18 but you still can't drink until you're 21). Once they hit that, aging slows way down until around 300, at which point the rate of aging will increase again until they die. 400 is equivalent to human 50-60, and 500 is like 70-80. Living to be over 550 is extremely rare.

There are some genetic oddities which are fairly common among dalar, much more so than humans. About 20-30% of dalar are left-handed, and aspirin/NSAID sensitivities are so common that such drugs are rarely used. The overwhelming majority of dalar (90% or more) either have no or very little sense of smell.

Diseases generally affect dalar the same as humans.

Another major difference between humans and dalar is in their mental structure. The dalar have a dual-mind structure, which is best described in computing terms. Basically, human brains are a single-core processor, while dalar brains are a dual-core processor. Dalar can't think any faster than humans can and they aren't any smarter than humans are, but they can think about two things at the same time. (more or less--it's not a perfect separation) If they're only doing or thinking about one thing, they'll think about it as fast as a human would, but if they're doing/thinking about two things, they'll think about them both in as much time as it would take the human to get done thinking about the first thing.

Birthrate
Given their extremely long lifespans, the dalar have very low birthrates. For various nebulously-defined reasons, not only do dalar women ovulate much less commonly, but it's harder for them to get pregnant even during the window of opportunity. Call it weird hormones or something.

However! Once a woman becomes pregnant once, it's significantly easier for her to become pregnant again in the few years following the pregnancy. Initially, this is balanced out by the lower odds of getting pregnant due to breastfeeding (which is very common), but it rises sharply compared to a woman who either has never been pregnant or who was last pregnant more than ten or twelve years ago. As a result, those dalar women who do have children tend to have them in "clumps" of three or four at a time, and may have multiple such groups of children. (which, coupled with the long lifespans, means it's entirely possible for someone to be older than some of their aunts and uncles!)

Siblings from the same "set" as you are referred to as hepin/wadas, loosely translated as "milk-brother/milk-sister", and siblings from different sets (either older or younger) are erate/erataha "blood-brother/blood-sister".

Half-Dalar, Half-Humans
Dalar and humans are genetically compatible enough to interbreed. Given the very low fertility rates of dalar women, this tends to be with a dalar man and a human woman. However, such hybrids are quite rare. First, dalar men have low fertility rates too. Second, the woman's body may reject the sperm or even have an immune reaction to it. Second, there are extremely high rates of miscarriages, due to the biological differences between dalar and humans, including both the needs of the growing fetus (slightly different from the needs of both dalar and human fetuses) and rejection by the mother's body of the embryo. Finally, because the gestation period for dalar is longer than that of humans (a full 12 months), half-dalar, half-human infants are generally born small and underdeveloped, to the point that a human doctor would likely assume they calculated the conception date very wrongly.

Complicating the physical factors are the social ones--dalar usually aren't very attracted to humans. While humans physically resemble dalar, the inability of humans to manipulate amati is uncomfortable to dalar. Furthermore, due to their long lifespans, many dalar focus on long-term relationships rather than short-term ones, and any relationship with a human is going to be "short-term" by dalar standards.

Furthermore, hybrids are almost guaranteed to have mental problems and occasionally physical ones, mostly because their brains don’t work quite like either humans or dalar. This may manifest as essentially untreatable schizophrenia-like or dissociative identity disorder-like symptoms, or various neurological disorders like seizures and partial or full paralysis.

Half-dalar, half-humans who survive to adulthood are almost certain to have longer lifespans than full humans, though not as long as full dalar. As explained by the Amati Exposure Principle, "halfsies" who are born to human mothers generally have shorter lifespans and less ability to sense and manipulate amati, while those born to dalar mothers have longer lifespans with dalar-like amati abilities. Thus, half-dalar, half-humans who are born to dalar mothers and raised among the dalar tend to have lifespans approaching 300 or even 400 years old, while those born to human mothers and raised among humans tend to have lifespans closer to 150 to 200 years old. In either case, they typically age more slowly than humans (appearing only 18-20 when they're 30 or older), then effectively stop aging for decades.

Socially, "halfsies" tend to be looked down on in dalar society. However, if a dalar encounters one in human society, they would quietly direct them to dalar assistance programs; they might be the gross black sheep of the family, but they're still part of the family.

Demographics
There are between 2 and 3 million dalar living on Earth. The majority live in one of three major dalar nations: Sanmra, Tuanmali, and Lorhan. The other half a million to a million live in smaller dalar nations or among humans.

History
Dalar were actually originally humans, transformed by a massive amati confluence that occurred in 1253 BC. (give or take) The initial change did not cause those affected to become dalar overnight, but rather opened them up to conscious control of and attunement to amati. Over time, this exposure led to increased lifespans, mild telepathy, and the other various abilities that dalar have today. The first generation of "dalar" lived unusually long, healthy lives and discovered the ability to sense the presence of other dalar; subsequent generations experienced increasingly long lifespans and increasingly advanced telepathic and amati-manipulating abilities until they reached their final form three or four hundred years after the original confluence.

This history is not broadly known by dalar; it is one of many theories about their origins, and is not even necessarily the most widely-accepted one. However, as the author, I get to say that this is the real explanation.