Warning: The following contains spoilers for My Hero Academia Chapter 333, "Specter," by Kohei Horikoshi, Caleb Cook, and John Hunt, available in English through Viz Media.
One of the qualities My Hero Academia celebrates in its Pro Heroes is their selflessness. No matter their personalities, when push comes to shove, most of them can always be counted on to do their very best to resolve any dangers they might encounter. Star and Stripe was only recently introduced but everything about her personality and exploits indicates that she’s as heroic as anyone else.
Star's very first appearance showed her displaying this very heroic quality. When All Might asked for assistance from the world's governments to deal with the All For One crisis, many of them were reluctant to send in the Pro Heroes for fear of sabotaging their own safety. Those who were willing to help were stuck dealing with the logistics of authorizing international action. Waiting for approval was too tedious a process for Star, especially when it was her mentor who requested it and she set off for Japan without gaining the proper approval.
It's important to note that this move was tantamount to career suicide for Star and Stripe. Unlike the superheroes in most comic book stories, the heroes of My Hero Academia's world are strictly regulated by their countries' governments. They receive compensation based on the amount of work they complete and only those with Professional Hero licenses are allowed to take on a Pro Hero's responsibility. By arbitrarily heading to Japan, Star forfeited her hero license and all the rewards that came with it but it didn't matter to her. When she was informed of her action's repercussions, Star simply said "Fine by me!."
She continued to display heroics during her battle with Tomura Shigaraki by taking on the brunt of the action herself. Even with foreknowledge of Tomura's fatal Decay Quirk, Star didn't shy away from a close-quarters battle, even baiting him into getting close to her at one point so she could land a powerful hit. After Tomura survived the Hypersonic Intercontinental Cruise Punch, she had a golden opportunity to destroy him once and for all before his Regeneration Quirk fully kicked in. But that would have forced her to blow up one of her teammates' jets.
The resulting explosion would have killed both her teammate and Tomura but Star's conscience did not allow her to sacrifice another's life. Even as he encouraged her to smash his jet along with Tomura, she refused, instead of allowing the villain to get close enough to grab her face. Star's decision at the time seemed misguided as it potentially placed the most powerful Quirk in the world in the hands of the most dangerous man alive but she had already prepared a contingency.
Again Star and Stripe's selflessness was put on display, in the few moments she had before Tomura made contact with her, she placed a rule on New Order itself, transforming it into a parasite that would destroy all other Quirks that shared the body it inhabited. Its effect was almost instantaneous as Tomura's body began bursting apart from the inside as soon as he tried to use New Order. She could have prioritized protecting her body from Tomura's Decay first before sabotaging New Order but her self-preservation would have spelled doom for the entire world.
Star's last-minute effort to protect herself from Decay happened an instant too late and even as she crumbled away she never expressed any regrets. She saw ending Tomura in exchange for her life as a worthy sacrifice and instead said a silent thanks to All Might for saving her life and allowing her to live out her dream of becoming a hero. Star's short-lived appearance described someone who always put the safety of others above her own. Her ultimate sacrifice has severely crippled All For One and may have given our heroes the leverage needed to win the upcoming war. Her ultimate sacrifice has severely crippled All For One and may have given our heroes the leverage needed to win the upcoming war. If that's not a hero, then no one else is.