Very few characteristics bind them beyond their size. From 5 to 10 centimeters, they can have just about any shapes and speak nymphs, just like all the other intelligent creatures of the Nymph World.
They have no gender, even though some might give that impression through their given shape.
Pixies who are in the right reproductive mood will start secreting a fine dust that will cover their bodies. If two such pixies start rubbing against one another, chances are that one or both of them will end up pregnant, a bit like mammals, although there's no breastfeeding.
Newborns will live off the fine mixed dust one or both of their parents until they are ready to be on their own, a month or two later.
Usually, the offspring’s physiology will look like a mix of both parents, but it's not a straight forward process. There might be large differences, to the point where the infant doesn't look like any of its parents even if they both were nearly identical.