(last updated: 12/28/17)
Anyone can be fast, but it takes a certain type of skill to be able to respond. Reflex represents the ability of a fighter to react and respond to the actions of other fighters. While the speed and difficulty with which an enemy's attacks may be repsonded to is a direct measure of how strong they are overall and has as such a direct correlation to Ratio, reactivity is a completely seperate measure, more dictated by one's style than their overall skill.
Overall physical speed is not always a great measure of a person's ability to elude or mitigate damage. A person who is physically fast may be able to make their attacks quicker in some way, but may not have good maneuverability. THere is a measured difference between rote memorization and muscle memory, and the true tactical prowess that goes along with being able to respond to an attack. Reflexes are the realm of a fighter who is able to track, plan and respond to an enemy's attack in real time. Such fighters can block or even elude an enemy's grasp entirely. As such, it is just as important a measure of one's mental faculties as it is their physical acumen.
Fighters who are higher in reflex may naturally be more aware of their surroundings, think faster, or even be physically faster. At medium levels of reflex, fighters can run up walls and leap onto lampposts, but at the highest levels of competency, fighters with great reflex can move and respond so quickly to attack that their opponents may think they're fighting ghosts, performing feats such as standing on walls on walking on water. Masters in reflex can often predict how an enemy will move, sometimes long before the enemy themselves knows. As a result of this, masters are able to trivially catch the bullets fired by untrained thugs and security guards simply by predicting where and when they are going to shoot.