If you're building a game, you need your game agents and characters to move on their own. A standard way of doing this is with 'steering behaviors'. The seminal paper by Craig Reynolds established a core set of steering behaviors that could be utilized for a variety of common movement tasks and natural behaviors. This Ruby library can accomplish many/most of those tasks for your Ruby / JRuby game. The basic behaviors can be layered for more complicated and advanced behaviors, such as flocking and crowd movement.
Gemfile gem 'steering_behaviors', '~> 1.0.1'
install gem install steering_behaviors -v 1.0.1