Thie name of this creature is derived from the superficially similar Luminous Siro ('siro', in the Toxiliviti language, means 'star'). Whether the two are related, if such a term can be used for Phenomenon class creatures, is uncertain, but tey share some base characteristics including a simple vermiform body plan, undifferentiated tissues, and the capability and preference for levitation.
Chromatic Siro are colonial animals, and an individual is better understood to be a group of several 'units' rather than a single bodied creature. An individual 'unit' is similar in shape to a legless lizard and about the length of a man's forearm. The body colouration is solid, unpatterned, and bright. Reds, yellows, greens, blues and magentas are all common, all highly, unnaturally saturated in tone. The anterior end bears a simple mouth and two narrow, black eyespots, which do not appear to be any more than simple markings. Dissection of the body reveals a soft inner flesh with no structure and the same bright colour all the way through. A simple blind-ended cavity begins at the mouth and runs through about three quarters of the body. No food or other items have been found inside, alhough the creature will often 'swim' with its mouth open as though attempting to swallow invisible objects. The only other structure within the body is a silvery blue-white organ shaped like an elongated teardrop, with the blunt end at the front above the mouth and the long end tapering down the back above the 'gut'. This organ's tissues, like the rest of the body, are homogenous throughout.
The units are never found alone. The smallest chromatic siro encountered on record was made up of three, while the largest was nine strong. They have never been observed mating or giving birth, though sometimes smaller units have been observed and it is hypothesised that two siro may exchange units on meeting.
Chromatic siro appear to be curious and playful, investigating anything in their environment by swarming nearby. Even their normal movements are energetic; the group members weave and dance around one another in energetic spirals. Even testimonials from the few people bonded to a chromatic siro describe "a joyous personality", as if even the most mundane of objects and situations are fascinating new discoveries. Over time this personality transfers to the human partner, and while it can be beneficial in the short term, in advanced cases they become unable to function independently, too busy gleefully investigating a wall or a table to go about their day to day lives.