Watch Jose Napoles mate, he wasn't an out and out defensive fighter but if you're wanting to improve defensive footwork whilst maintaining your generalship in the center of the ring as opposed to moving excessively around it, then Jose Napoles is your man, or Bernard Hopkins. The footwork of Hopkins has been perfected, it's with scientists such as Hopkins that we can differentiate between great footwork and great mobility, for example Roy Jones was very mobile, but his footwork from a technical standpoint doesn't have shit on Bernard Hopkins, who was deceptive with it.
Anyway, Napoles and Hopkins are both exponents of complete footwork in my opinion, but if you're looking to just improve your lateral movement, side to side, then Luis Rodriguez is that guy to study imo, when he keeps it on the outside with his lateral and circular footwork, it's no contest, and that's against one of the all time greats