Besides Soft Ones, Hard Ones are the only lifeforms inhabiting a small rocky planetoid located in a parallel universe, where the nuclear interaction between particles is much stronger than in our own. Unlike Soft Ones, which are gaseous beings with three different sexes (Rationals, Emotionals and Parentals), Hard Ones are solid and have no sexes. It has been implied that their bodies are denser than rock, and they are known to feel pain if a Soft One pass through them. Their shapes are fixed and implied implied to be largely humanoid; there is even a mention of them having an analogue of a smile.
Soft Ones respect Hard Ones and recognize them as more intelligent beings. Hard Ones, on their side, are usually friendly and caring towards Soft Ones, helping them to find suitable partners and acting as teachers to young Rationals. They are, however, very mysterious and hardly ever talk about themselves. The total Hard One population is about 300 (about one thirtieth of the population of Soft Ones). Like Soft Ones, they dwell in caves, but unlike them, hardly ever go to the surface, not requiring sunlight for sustenance.
Some Soft Ones believe that Hard Ones get energy from the rocks, rather than the sun, and that they never die. This is not true, however: although Hard Ones have a very long lifespan, they are not immortal. The fact that, with their sun slowly dying the population of Hard Ones has also diminished through the years, seems to prove that Hard Ones do feed on sunlight, not unlike Soft Ones.
The biggest mystery concerning Hard Ones is their reproduction, since no young Hard Ones are ever seen. However, it turns out in the end that Soft Ones are actually the immature stage of Hard Ones. They start life as Soft Ones, knowing nothing about their destiny. After having reproduced and generated three babies - a Rational, an Emotional and a Parental - the triad of Soft Ones melt together and "pass on" to become a single individual. That's how a Hard One is born. However, this fact is carefully hidden from Soft Ones, who believe that when they pass on they simply die. The reason why Soft Ones are not told the truth is that Hard Ones believe that only when the Soft Ones remember their Hard One periods by themselves can they choose the proper time for the permanent melting.
A technologically-wise race, Hard Ones are able to obtain energy by exchanging matter between their universe and ours - a process known as "pumping". Although they are a very intelligent race, they haven't evolved into a space-faring civilization. Nor is there much point for them to; all the stars in the universe are dying, so there is little to be found outside their world.