In Rome's Revolution, the first thing Rei notices about Rome is that her eyes seem to glow, even in the dimmest light. This is because the Vuduri have an extra reflective layer of tissue behind their retinas to reflect light forward. The completes the Maksutov-Cassegrain reflector, built into their eyes which gives them telescopic vision.
From Wikipedia: The tapetum lucidum (Latin: "bright tapestry") is a layer of tissue in the eye of many vertebrate animals. It lies immediately behind the retina. It reflects visible light back through the retina, increasing the light available to the photoreceptors. The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores that hunt their prey at night, while others are deep sea animals. Eyeshine is a visible effect of the tapetum lucidum. When light shines into the eye of an animal having a tapetum lucidum, the pupil appears to glow. Eyeshine can be seen in many animals, in nature and in flash photographs.
Rei finds the effect mesmerizing. The Onsiras Zengei (drones), half machines, half men, do not have a tapetum and their eyes are very dark, almost black. They are occasionally referred to as "shark eyes." The Reonhe and Broncase, the "royalty" of the Onsiras, have one dark eye and one light eye. Estar, the villain in Part 1 and Part 3 of Rome's Revolution, is a Broncase as is Sussen, the spy on Deucado who precipitated the crisis of Book 3. Sussen returns in Rome's Revolution, Part 5 entitled "Rome's Evolution."