Reflex is the main stat used in avoiding attacks. A higher Reflex makes you much more likely to succeed at a dodge attempt. Reflex also factors into how good your chances are to block or endure, though not as directly as it influences your chance to dodge. It has no influence on interrupting.

A character with a high Reflex score is very agile, and may have a 'slight' build to them, or perhaps even be outright tiny. Most female characters and males like Choi are excellent examples of how Reflex is partially tied into how big or small you are. Likewise, really big people like Zangief, Daimon, and Daigo tend to have lower Reflex speeds not only due to their lack of agility, but also due to their great physical size.

Reflex can be used as a rough suggestion or measure of someone's maximum running speed, too. However, Reflex primarily measures your short-range agility and ability to react to incoming attacks, which is also largely derived from experience. Thus, while Ken has a higher Reflex score than Mai, it's entirely likely that Mai can outrun him in a foot-race.

Thus, just as a character with a lower Physical score can be more muscular than someone with a higher Physical score, a character with a lower Reflex score may be a faster runner than someone else. Use some judgement in this matter. Cammy might actually be able to catch up to an expert bicyclist, for example, despite the fact there's others with higher Reflex scores.