My Hero Academia takes place many years in the future, where superpowered Quirks are the norm and costumed pro heroes regularly battle criminals and villains on the streets. These heroes are like celebrities, unlike the Punisher or Batman, but they must still keep their true names a secret, and the villains do the same. That information is vital to their survival.
Hero names such as All Might, Endeavor, Hawks and Mt. Lady are household names, and in the criminal underworld, names such as All For One, Dabi and Overhaul are equally famous. By contrast, the yandere villainess Toga Himiko uses her real name, refusing to use a moniker or code name during her criminal career. This ties into her personal themes of authenticity and self-expression.
Toga Himiko's Quest For Personal Authenticity
Many heroes and villains in My Hero Academia thematically tie into the narrative in terms of the subjective meaning of "good," whether or not people's talents should be regulated, how to handle people who feel ostracized and much more. Toga Himiko the villain represents the quest for personal authenticity in a world where some people must hide their true nature because of their Quirks.
Unfortunately, many people are mistrusted, feared or made into pariahs because of their heteromorph appearance or their gruesome Quirks, and Toga Himiko struggled with her own Quirk in similar ways. She has a Quirk fueled by blood, which in turn led to her disturbing fascination with blood, including bird blood. Even her parents rejected her, demanding that their daughter be the "normal" child they always wanted.
Himiko strained herself to appear normal and keep up appearances, but after graduating junior high school, she snapped. Himiko drank a classmate's blood and went rogue, and soon idolized Stain the hero killer as her role model. Along the way, Himiko designed her own villain costume, including a school uniform and a blood collection apparatus, which all ties into her Quirk and theme alike. Himiko is clearly determined to be true to herself, and thus wears a school uniform to subvert society's expectations of a proper schoolgirl while also continuing to use her real name.
Unlike her League of Villains allies such as Spinner, Twice or Dabi, Himiko simply went by her real name, introducing herself that way to Overhaul and even some of U.A.'s students. Himiko has already spent a lifetime trying to be someone she's not for the sake of others, and she is fed up with false exteriors and lies. She must be true to herself to the very end, and she'd sooner perish as Toga Himiko the villain than be captured and forced to assimilate back into society. She is declaring her true nature loudly and proudly, and no one can take it away from her.
How Himiko's Theme Ties Into Todoroki Shoto & Shigaraki Tomura
Notably, Toga Himiko isn't the only character to struggle with themes of self-expression and authenticity. Her boss, Shigaraki Tomura, was born as Shimura Tenko, but he was forced to give up that identity after his Decay Quirk activated in his boyhood, and All For One renamed him. The name Shigaraki Tomura is a mask that Tenko uses to force himself to keep up his villainous career and remind him of his duty to serve All For One. No doubt this conflict is highly stressful to him, even if he revels in the power that All For One and Dr. Garaki have given him.
Most of all, Himiko shares a naming convention with Todoroki Shoto, whose hero name is simply his first name written in katakana. This contrasts sharply with names like Froppy, Deku and Uravity, and at first, it seems anticlimactic for Shoto to call himself Shoto as a hero. However, Shoto actually has similar reasons as Himiko to cling to his real name and refuse to wear a mask, literally or otherwise. He was born from a Quirk marriage to be Endeavor's ultimate tool, but Shoto is fighting hard to escape his father's clutches and forge his own identity and destiny.
To that end, not only did he reject his father's fire, but he also reasserted his true identity with his official hero name, unwilling to hide who and what he really is. He's not just the talented son of Endeavor and he's not someone's tool. He is Todoroki Shoto, nothing more or less. In this way, he reclaimed his independence and sense of self, just like his villainous counterpart Himiko.