Sega Master System / Mark III / Game Gear
Press + on controller one (jumping reverse kicks) at least ten times on the first screen of level four before either player moves.
On the last level, use the unlimited continue trick. In the last room before Jeff, move up and down until the enemies on the screen do the same. Continue to move up and down and make your way to the other side of the room. Be sure all enemies are off screen. Now press down and fall into the spikes, this will kill you, but with infinite continues no big deal. Keep doing it until new enemies appear and repeat till Jeff comes out. Defeat Jeff on the very right side of the screen. Once defeated you make your way to your girl-friend. But... instead it will either be an Abobo or Linda. You proceed to kiss the enemy but in a fairy-tale style the enemy turns into your girl-friend.
See the main page for Double Dragon (ダブルドラゴン / 双截龍)