Soft Continuum manipulators are increasingly popular because of their ability to interact safely with humans, maneuver around obstacles and enable cost-effective operation. In this paper, we investigate a unique manipulator composed of two asymmetric fiber reinforced pneumatic building blocks, one capable of bending and the other axially rotating. The combination of these building blocks can yield spatial motion including helical deformation pattern. While this design architecture is beneficial for whole arm manipulation and grasping, the asymmetric combination introduces structural coupling between the two modes rendering an analytical model hard to formulate. In this paper, we propose a Cosserat rod model to capture the deformation of the manipulator whose material properties and actuation parameters are estimated through extensive experiments. Once the model parameters are determined, the Cosserat model can be used to estimate deformation profile even in the presence of external loads. Such a framework is general and can be applied to obtain the forward and inverse kinematics of any continuum manipulator.