Planet of Origin: Mars
Martians are a humanoid species living on Mars which is genetically and socially compatible with Earth's humans but which possess "cold, analytical reasoning powers" but lack the "emotional drive and boundless energy of the Earthman".
After mankind began to travel to the red planet, tweenies - Half-human half-martians half-breeds - started being born (see picture). Thet weren't accepted on Mars and so had to be brought back to Earth where they were being taken care of by social services. mainly in asylums. Tweenies are generally more intelligent than humans or martians but look like normal humans with a "brush of wiry, dead-white hair that rose stiffly in all directions like porcupine-quills".
Note: Martians are actually never met in the stories conerning Tweenies but are believed to be human-looking as the stories tell of more than a thousand tweenies are known to have been born, so inter-species unions must have been somehow not against nature, although no details are given on that matter.
Source: Isaac Asimov, The Half-Breed, Half-Breeds on Venus