After you have finished entering the characters for a character's name, highlight END (the Japanese word in the bottom-right corner). While holding start, press button 2. If done correctly, the next character will appear.
In battle 16, search the Cactus just to the left of the force's starting location; Sasuke will then automatically join the force.
In battle 19, search the Grass in the southeast corner of the battlefield; Kojirou will then automatically join the force.