Ronin are not a playable faction in Bushido, though it is possible to build a Warband composed of only Ronin profiles as long as it is based on a faction which permits all the profiles you wish to use.
The Ronin descriptor is applied to model Profiles, Special Cards and Themes which are not faction aligned, and are therefore available to be used by more than one faction. This could be mercenaries brought in for a specific task, friendly (or unfriendly) Kami spirits, a personal model or Warband upgrade (Equipment, Event etc..), or a widely understood and accepted battle tactic (Theme).
There are no specific play styles or strengths and weaknesses that apply to all Ronin, so these sections are not included as each Ronin card needs to be assessed on its own merits.