1. A computer-readable storage medium having a game program stored therein, the game program causing a computer of a game apparatus, which generates a game image for display on a display screen, to implement a method comprising: a game image display control step of allowing a game image comprising one or more game characters to be displayed on the display screen; a coordinate detection step of detecting a coordinate value at specified time intervals, the coordinate value indicating a position on the display screen which is pointed to by a pointing device; a shape identification step of identifying a graphical shape of an input trajectory represented by a coordinate value group detected by the coordinate detection step; a size calculation step of calculating a size of the graphical shape of the input trajectory represented by the coordinate value group detected by the coordinate detection step; a basic condition determination step of determining an attack style for changing respective characteristic parameters of at least one attacked game character based on the graphical shape identified by the shape identification step, each characteristic parameter indicating a characteristic of the corresponding attacked game character; an effectiveness determination step of determining an effectiveness of the attack style on the at least one attacked game character based on the size calculated by the size calculating step; and a characteristic parameter change step of changing the characteristic parameter of the at least one attacked game character based on the attack style determined by the condition determination step and the attack effectiveness determined by the effectiveness determination step.