Every single trait from a Nihilo X hybrid in birth appears as a full Nihilo X, but eventually changes appearance to suit the hybrid. If a Nihilo X breeds a child and it does not look completely like a Nihilo X, then it is a mutation and highly unstable. The unfortunate parents either have to attempt to raise a creature unlike any other as a child, set it free in the wild, or put the poor thing down. This is an unfortunate circumstance, but such is the life of the Men from Nothing.
As they were clueless to everything but language when they arrived to us from wherever they came, they trusted the people that they first met. We built homes with them, created games with them, et cetera. They adapted to their first lifestyle quickly, and it ended up being an amazing alliance between a dark beast and humans, and unlike demons, this had no moral repercussions, as the Nihilo X species rarely goes against what they are told to do. Nihilo X are extremely intelligent, but many are easily manipulated due to their trusting disposition.
Nihilo X have the ability to cause hallucination, vomiting, mental trauma, and other types of negative effect towards living subjects of their attacks. Their physical stature is strong, but when attacked, they crumple at light attacks. While this has exception, the exceptions are most likely to happen once you enrage a Nihilo X, as they gain stamina, physical prowess, magical ability, and defenses unmatched by much. They are nigh indestructible in the right conditions unless you attack them with their special combined elemental weakness, which varies per Nihilo X. However, extremists against the species showed us that it is not difficult to find their elemental weakness, which makes the Nihilo X fade into oblivion.
Nihilo X do not need to breathe or consume. Instead, they get nutrition and self-restoration from being in dark places. This sprouted a long debate whether these men were men at all, or demons from another dimension hellbent on our destruction. This appears not to be the case, as being smart, sensible people (OK, I can barely keep a straight face with this one), we brought them to a prominent species in society. This has been said to be a mistake, but I say that, even if it was, it was a damned good mistake in the end.