When playing Risk of Rain, it may be preferable to use a game controller, game pad, or joy pad.
According to Steam the game has 'partial controller support' which simply means the game does support a controller but you may not be able to play the game without a keyboard and mouse.
It should be noted that as of Update 1.2.1 the game can be played with only a controller although it is still easier to navigate menus with a mouse.
Microsoft Xbox controllers are supported by default with Xinput as of the 1.2.1 update, but DirectInput is still not supported. In order to use a controller that utilizes DirectInput, the developers themselves recommend Xpadder
A list of programs to help you emulate an xbox360 controller are listed below: