Yehr'Avali, AKA "half-elves", are a rare, rare phenomenon. Elven fertility is tricky, and sparse at best. A female aval can bear, at most, 4 children in her life, and few bear more than 2...squandering her womb on a half-breed is a great risk. This is not because of any stigmatized notions of "avali purity", but because yehr'avali are sterile, and thus a dead-end for any woman. Conversely, human females can rarely bear a half-elven child to term. And of course, there are no children of the yehr'avali themselves.
There are no physical features that set yehr'avali apart from their parentage. Generally they do have pointed ears like an avali, and might resemble an avali as much as any child resembles either parent. They also may or may not inherit the metabolism, vision, and slanted eyes. Their lifespan varies wildly. Half-elves can be shorter-lived than human, but they have also known to live longer than the oldest avali (hybrid vigor, baybe!).
Half-elves have as much aptitude for avaian magic as pure-blood avali, but their chances of having any at all are sketchier. They are also capable of possessing the "simple magic" that occasionally develops in humans.
There is no yehr'avali culture, really, as there aren't enough of them to form one. They are welcome amongst the avali, though are generally pitied (children are a joy to the avali, so sterility is considered a tragedy), and can find their place amongst humans just as easily. Some cultures look very favorably on them, whilst others are suspicious...it generally depends on the opinion of said culture for the avali.