Shadow Man (or Shadōman in Japan) is one of the 8 Robot Masters and main antagonists of the NES video game Mega Man 3 as well as one of the 4 Robot Masters and main antagonists of the Gameboy game Mega Man III. Unlike most of the Robot Masters throughout the Mega Man series, Shadow Man was not originally created by Dr. Light. Rather, Dr. Wily found Shadow Man deactivated before he reactivated and turned him into one of his 8 robot masters.
While not much is known about Shadow man's origins, it should be noted that he is implied to have been initially created by some sort of extraterrestrial society. The "metal" on his body has been stated to not be composed of any material found on earth. While it has never been explicitly stated where Dr. Wily found Shadow man, the former did so regardless immediately sensing the latter's potential.
Mega Man 3
After reprogramming and repurposing the recently discovered Shadow Man into a combatative Robot Master, Dr. Wily assigned him to a respective base of operations along with 7 other reprogrammed androids, Shadow Man being unlike them in the sense that the other 7 were stolen and originally created by Dr. Light to be reprogrammed by Dr. Wily into the Robot Masters in question presumably following his defeat after the events of Mega Man 2. Mega Man infiltrates Shadow Man's said base to defeat him and take one step closer to defeating Dr. Wily. If the player has defeated Top Man and acquired his titular Top Spin, Mega Man would have an advantage against Shadow Man due to the Top Spin being Shadow Man's weakness. The Top Spin is Shadow Man's weakness possibly because during the attack the player would be moving rapidly enough to keep up with the former and could also possibly be taking him off guard. Ultimately, Mega Man defeats Shadow Man and obtains his Shadow Blade, with Shadow Man never appearing for the rest of the game.
After Mega Man defeats all of the Robot Masters, Doc Robot takes over 4 out of the 8 Robot Master's bases including Shadow Man's. In this stage, Shadow Man is replaced by Doc Robot assuming the form of Wood Man, in addition to Doc Robot taking on the role of Heat Man. However, Doc Robot/Heat Man is weak against Shadow Man's Shadow Blade, as well as Shadow Man's weakness, Top Man's Top Spin.
Mega Man III (Game Boy)
Shadow Man returns as one of the 4 main antagonists of Mega Man III. Shadow Man's role is almost identical to his role in Mega Man 3, the only notable difference being that Shadow Man's weakness in this title is actually Gemini Man's Gemini laser. The weakness change is because Top Man isn't in this game, but also possibly because the laser would be able to keep up with Shadow Man's speed. Just like before however, Mega Man defeats Shadow Man for the latter to never appear for the rest of the series as an in-game boss although he would go on to appear in non-canon titles. Canonically speaking though, Shadow Man is presumably defeated for good.
Super Smash Bros. Ultimate
Shadow Man appears as a support spirit that grants the user a Killing Edge at the beginning of the battle. His spirit battle is a stamina battle against three Greninja on the Battlefield form of Norfair with increased power to their Water Shuriken special.
Shadow Man has the ability to conjure robotic frogs. This is possibly a reference to the mythological ninja Jiraya, who could manipulate frog magic.
Shadow Man is the only Robot Master in Mega Man 3 (aside from Doc Robot/Heat Man) to take damage from all special weapons in the game, with different weapons doing various amounts of damage respectively.
Shadow Man is one of the only bosses throughout the Mega Man series to be able to slide. The others being Saturn, Copy Mega Man, and Copy X.