WARNING: The following article contains discussion about sexual assault.
The Rising of the Shield Hero follows the story of Naofumi Iwatani, a shut-in who has been summoned to another world as the titular Shield Hero. Alongside the Spear, Sword and Bow Heroes, Naofumi was tasked to fight off the Waves of Calamity. He started as an idealistic hero like the other three, but his character took a drastic turn after being betrayed by Princess Malty, suddenly becoming a moody and selfish antihero.
Being hated by most of the kingdom of Melromarc's population, Naofumi was forced to roam in exile without any support. His general distrust and apathy toward others' opinions largely helped his survival. Fortunately, he met the demi-human Raphtalia and the giant bird Filo, who both aided in suppressing his darkest thoughts. But before Naofumi met his trusty aides, he had Princess Malty as his sole companion.
When nobody chose to be his companion, Malty volunteered to join his ranks. The traitorous princess hid her true identity and pretended to be on Naofumi's side. She acted like she was guiding the naive Shield Hero, only to swindle his money and equipment later on. Princess Malty invited Naofumi to drink with her -- a likely ploy to frame the hero -- but when Naofumi rejected, she executed her plan in a different way. She went to Motoyasu, the Spear Hero, and tearfully accused Naofumi of sexual assaulting her.
The morning after, Naofumi discovered his money and armor to be missing. While running around looking for Malty, he met a bunch of knights and sought their help, but was arrested instead. Princess Malty revealed her betrayal in a spiel of how the Shield Hero supposedly assaulted her, and the evidence that nailed Naofumi's coffin was a planted lingerie. After that he was banished and his name tarnished, transforming him into the currently dark and cynical antihero Rising of the Shield Hero viewers are most familiar with.
Malty's controversial plot raises the question of her motives. The first that comes to mind is the temptation of money as she easily coveted Naofumi's slightly-higher initial funds, though this reasoning is dulled by the fact that Malty is a crowned princess. Another speculation is the princess being naturally hateful. While that is true to a certain extent, there are other major factors in play.
The Rising of the Shield Hero later reveals that despite being the first princess, Malty is only second-in-line for the throne due to being a problem child, which further amplifies her already-hateful personality. Her jealousy even reaches the point of wanting to kill her own sister. Malty is also a firm believer of the Church of the Three Heroes, who branded Naofumi as a demon, and has close ties with the church's leader who feeds her ego with false information about the Shield Hero. Malty's nasty behavior mixed with the church's influence and her raging jealousy prompts her to constantly obstruct Naofumi's journey and attempts to get stronger in the vilest of ways without a hint of remorse.
Princess Malty pulled off one of the nastiest betrayals in the world of anime. Even though her spiteful behavior plays a major part in her growing desire to destroy the Shield Hero, her social status and religious beliefs are also to blame. Having the influence to pull strings and moral validation of her evil deeds transformed her into the irredeemable villain of Rising of the Shield Hero.
