The de-facto protagonist of the Double Dragon series. Successor to the Chinese martial art of Sou-Setsu-Ken. He began his Chinese martial arts training at the age of 12 and went on to master several fighting styles until becoming the Sou-Setsu-Ken successor at the age of 20. Sou-Setsu-Ken is a fighting style that combines the techniques from all sorts of martial arts such as Karate, Shorinji Kempo and Tai Chi into one school.
Adaptation Dye-Job: Blond in the arcade games and Neon, brown hair in the NES games and black hair in the animated series.
Early concept/promotional art for the original game depicts Billy not only as the darker haired brother, but has him wearing the red outfit instead of blue, which implies that he might been originally intended to be the Player 2 character (before the identities of the Lee brothers were fully established).
The younger Lee twin, who helps his brother Billy fight the Black Warriors, while being secretly in love with Marian. In the NES version, he is actually the Shadow Boss, the true leader of the Black Warriors, although this plot point is swept aside in the sequels (which brought back the co-op play missing in the first NES game).
Adaptation Dye-Job: Brown hair in the arcade games and Neon, blond in the NES games and animated series.
Much like Billy, some of the early promotional art depicts Jimmy wearing blue instead of his usual red.
Composite Character: In the first NES game, not only serves as the new final boss, but essentially replaces Jeff (the head-swapped Lee brother boss from the arcade version) as the boss who uses all of the player's moves.
Billy's girlfriend, who used to be an assistant instructor in their old martial arts gym, but is kidnapped by the Black Warriors. In Double Dragon II, she is seemingly killed off in the beginning, with her ultimate fate varying depending on the version.
Faux Action Girl: If we are to believe the back-story given in the original game's official soundtrack and some of the console versions, she was formerly an assistant instructor in Billy and Jimmy's old dojo. Seeing how she goes down with only one punch to the gut in the opening of the original game without putting up much a fight, her qualifications for such a job are debatable. Averted by her Neo-Geo incarnation, who does have actual fighting skills.
Groin Attack: Gives Skullmageddon one after the end credits in Neon.
Badass Mustache: Abobo's in-game sprite sported a horseshoe mustache in the original game. The character illustrations for the NES version gave Abobo a clean-shaved look though, giving him an appearance similar to Kratos or Sagat.
Canon Immigrant: At first he was just a character made up for the NES version of the first game, but later appears in the arcade version of II as a stick-wielding fighter. His name was used for the Player 2 version of Chin (shortened to "Taime" due to space limit) in the arcade version of 3.
The leader of the Black Warriors, a biker gang leader whose organization grew in power after the nuclear war. Serves as the final boss in the arcade versions of the first two games, where he shoots Marian to death in the second game. In the NES versions, he's merely a figurehead, being the last opponent fought by Billy before the final battle against Jimmy in the first game and a complete no-show in the second.
Big Bad: In the first two arcade games and most remakes of the original.
Demoted to Dragon: In the first NES game , where he's just the last guy the player fights before the final fight with Jimmy. He's completely absent in the second NES game, although he does make an appearance in the PC Engine remake.
Sudden Name Change: To "Roper" in Battletoads & Double Dragon. The manual for the Master System version also switched his name with Jeff's.
Butt Monkey: Williams is most definitely one of these in Neon: Skullmageddon threathens to dock his pay for the monitors you break in levels 3 and 4 and 2 of his quotes when he kills you are "No dogfood for Williams today!" and "Guess who's sleeping inside tonight?"
Suspiciously Similar Substitute: To Abobo in Double Dragon II. In fact, he's literally Abobo with mop hair (same facial structure in both, the arcade and NES versions), to the point that the Mega Drive version's manual identifies as such (rather than as "Bolo") on the enemy list.
Blind and the Beast: Jawbreaker befriends the Samantha (Commissioner Briggs' blind daughter) after kidnapping her under the Shadow Master's orders in "The Sight of Freedom". When Icepick orders him to "eliminate" the girl, Jawbreaker rebels against the Shadow Warriors and rescues her.