Masako Yamashiro is the daughter of Ichiro Yamashiro and the sister of the late Oda Yamashiro. She saved Nate Heywood when he accidentally arrived in Feudal Japan and assisted the Legends in fighting Shogun Tokugawa Iemitsu. Although she and Nate shared an infatuation with each other, Masako urged Nate to move on, Nate that as a time traveler, he would have to leave her time period.
Masako and her father Ichiro were encountered by Nate Heywood and the Legends during their trip to Medieval Japan. Masako had lost her brother to the evil shogun and was to marry the shogun against her will. Masako fell in love with Nate after healing him. During the battle, Masako wielded her brother's former katana and helped the Legends in their battle against the shogun.
After the battle, Masako bid farewell to Steel with a kiss, and Steel convinced her to learn to use the katana and defend her village. It was also revealed that Masako and her father are ancestors to Maseo Yamashiro.
Altruistic, kind and brave, Masako is an all-around good person. She saved and healed Nate Heywood, despite knowing the society's prejudice against a gaijin, as well as the dangers of harboring them under the tyrannical rule of Tokugawa Iemitsu. At first, without any hope for help, Masako was ready to accept her forced betrothal with the Shogun, knowing that he'd punish the whole village for disobedience otherwise. She showed cunning and deceit while dealing with the Shogun's Samurai, putting on a false smile and making them think her a willing bride, trying to hide the gaijin's appearance. Masako later showed herself to be extremely brave, coming to the rescue of Ray Palmer and fighting the Shogun with the Yamashiro family katana. She grew infatuated with Nate Heywood, respecting him for being "hardcore" in trying to fight a samurai without any armor of his own, and later coming to her rescue as Steel, defeating Iemitsu. However, she was pragmatic enough to know that a romance with a time traveler was more or less impossible, telling that to Nate, before bidding him goodbye with one final kiss.
- Skilled swordswoman: Masako was proficient with the use of Yamashiro family katana, however, she was bested in combat by Tokugawa Iemitsu.
- Yamashiro family katana: This katana was passed throughout Yamashiro family, eventually ending up in the hands of Tatsu Yamashiro.