Reincarnated as the Hero’s Childhood Friend, Then Saving the World

Margarita is unexpectedly reincarnated into Sword Dance of the Star-Spun Fates, a game she once played. But upon opening her eyes, she discovers she is not the protagonist—she is merely a bit of background NPC cannon fodder. "I don't want to die!" Driven by this fierce resolve, Margarita fuses with the Star-Spun Stone, which serves as the Hero System, gaining an invincibility mode that can only be used for five minutes at a time. Together with the protagonist, she embarks on a journey to slay the Demon King. ...Wait, why does it seem like all the original heroines are trying to set me up with the protagonist?