To be honest, nobody really knowsIt's funny because that's actually true. Although the word "synthetic" means a combination of various things and/or something man-made, in Spore, that definition is a little different. With everyone's different ideas and interpretations of what they deem being "real", these characters that have no real shape or form, they are just purely made-up through imagination and exist "just because they do." So as a result, characters like this tend to be very odd-looking, as they were created to be "cool looking," but only really to the owner of said character. A benefit of this is that any synthetic character can have all sorts of weird and/or wacky abilities, as the laws of physics don't usually apply to them. Though it is preferred if these sorts of characters try to restrain themselves from causing too much (and I use that term lightly) chaos, as things can go from normal to flat-out insane in a matter of minutes. Another point about synthetic characters is their body structure, As it was mentioned earlier that characters of this archetype are made up of what-over there owner wants them to be made of, meaning they can drastically alter their shape and form at any given moment, in order to suit their owner's sense of humour (more or less, really). To add to it, synthetic characters can also alter the structure of other characters, but this will only last for as long as they remain in contact with said character. In short, synthetic characters try to have fun in their own weird ways, without causing as much damage (both physically and mentally) to their surroundings.
Members of this Class
Tenguman easily fits into this category, and is likely the first character to do so. 99Hedgehog could also be classified under this archetype, though he is more humanoid than synthetic (if you would consider a stick-figure to be humanoid).