A controller and a control method are capable of improving safety by automatic emergency deceleration action while suppressing a motorcycle from falling over. One arrangement also obtains a brake system that includes such a controller. In the controller, the control method, and the brake system, a control mode that causes the motorcycle to take the automatic emergency deceleration action is initiated in response to trigger information generated in accordance with peripheral environment of the motorcycle. In the control mode, automatic emergency deceleration that is deceleration of the motorcycle generated by the automatic emergency deceleration action is controlled in accordance with a lean angle of the motorcycle.