The present embodiments relate to a steering assist device and method, and a steering system. The steering assist device comprises: first and second input-side steering control modules for controlling an input-side steering motor so that an input-side instrument connected to a steering wheel is assisted; and first and second output-side steering control modules for controlling an output-side steering motor so that an output-side instrument mechanically separated from the input-side instrument and connected to a wheel is assisted, wherein the first and second input-side steering control modules and the first and second output-side steering control modules arbitrate a role assignment and can control a corresponding steering motor from among the input-side steering motor and the output-side steering motor according to the role assignment arbitration result.