Half-lives are essentially creatures who are born of powerful desires.
Half-lives are created when someone has a desire or wish so powerful. Once born, they carry a purpose throughout their life. Some half-lives don't know what their purpose is, but all seek to someday fulfill that purpose, or so it has been said. The problem is that when a half-life does fulfill its purpose, it loses its purpose, thus having no reason to be, and so are said to cease to exist.
It has been said that half-lives are born from the dead. This means that they are either re-incarnations of previously dead individuals or something else.
While most half-lives generally look monsterous, they do usually seem humanoid in shape, and at least once a half-life has been human enough to walk amongst people without raising suspicion. Additionally, if the individual making the wish that creates a half-life has some concept of what the half-life might look, or has a mental image connected to their wish, the half-life will take the form of the conceived image.