It is later revealed that Medusa's incarnation in the remake is in fact, Miyoko's mother, Ms. Mangetsu. She made a deal to Demaon to save her daughter, but at the costs of having her soul taken away by Demaon and turn her into a demon.
Nobita's Great Adventure into the Underworld
In original version, Medusa is just the skeletal-looking demon with hair full of snake whom sent by Demaon to hunt down and neutralized Doraemon and Nobita. It chased them through the time tunnel and finally petrify them. After they fell onto the tree at the lot, it returned back to the present time and never seen again afterwards.
Nobita's New Great Adventure into the Underworld
The woman whom became Medusa married with Prof. Mangetsu and together have child that they named Miyoko. For a long time they lived happily until the day Miyoko mysteriously became ill due to a deadly disease. Sadly, there's no cure for such disease. Mrs. Mangetsu was desperated so much that without thinking twice, she decided to sought the ancient spell from an ancient book to save Miyoko, and came up with forbidden spell where one would surrended his/her soul to Demaon in exchange of their wishes granted. When Prof. Mangetsu discovered this, he was horrified and warned her about other consequences that would happened.
Unfortunately for him, Mrs. Mangetsu uses that spell anyway, resulting her consumed by demonic flames and taken away by Demaon. Prof. Mangetsu tries to stop this, but it's too late though Miyoko eventually cured from her disease.
Events in the film
Gazing through his crystal ball, Demaon witnessing Doraemon, Nobita, Suneo, Gian, Shizuka, and Miyoko would come to foil his plans. So, he sent Medusa to see how powerful they are. He also sent the Monkey Demon both as her companion and to keep an eye on her in case something goes unexpected on her.
Medusa made her presence known to her former family by destroying her former home with a pillar of light. Though they survive the mystical blast, she had a demon to capture Prof. Mangetsu whilst cursed Miyako into a cute-looking mice (Nobita and his friends managed to tamper the curse by made her turned into cute anthropomorphic cat instead later on), before posed as her to manipulate Doraemon and co. She played the ruse well as when she fought against a remaining demon soldier whom showed up and tried to kill Doraemon and Nobita (though in the fight, it was Doraemon whom defeat the demon in the end). Once she and Doraemon and co. gained the possession of Nirnaeth's scroll, she removed her disguise and buried her enemies under conjured avalance. Fortunately for Earth, Medusa only gained half of the scroll, that resulting her being punished by Demaon.
After the ill-attempt to assassinate Demaon, Medusa quickly followed Doraemon and Nobita through the time tunnel and managed to catch up. She emerged from Nobita's desk and chased them through the sky of Tokyo. After a quite troublesome chase, Medusa petrified Doraemon and Nobita which the former fell back into Nobita's room while the latter into the tree at the lot. Succeeded in her mission, she returns back to the present time.
Demaon then gave Medusa her final mission, to destroy the seal on the moon in order to finally allow the demons to invade Earth, knowing that despite her resistance toward the magic of the moon, she will perish when the seal is destroyed. By the time she goes to destroyed the seal and Doraemon and co. showed up, something unexpected happened: Miyako's voice as she begged Medusa to stop caused Ms. Miyako's humanity to resurfaces, which made her briefly lose her identity in spite of able to resist it enough to destroy the seal. By the time the seal of the magic of the moon destroyed, the released energy not only killed Monkey Demon, but also destroyed Medusa's demon persona and free her original human psyche as she revert to her former human form. Prof. Mangetsu and Miyako was horrified with this, and as they held Ms. Mangetsu, Ms. Mangetsu revealed that because her soul was taken and turned into demon, the loss of her demon part made her cannot exist in her human form for long. In her last breath, she revealed that Demaon has put his heart into red moon of Magic Planet, which explained the first attempt to killed him failed, before finally disappeared by breaking into pieces then her locket fell to the ground. then Miyoko kept it then keep fighting with Doraemon and Gangs until finally killed Demaon. Then suddenly, the locket of Miyoko's mother flashed a beam of golden light and suddenly disappeared forever which healed the moon, protecting the Earth forever.