Catherine Aiyama, also known by her Radam designation Teknoman Sword, is a character from the anime Tekkaman Blade. Her original Japanese name is Feng-Li, and she is the former fiancée of Kengo Aiba, who is the older brother of the protagonist, Takaya Aiba. Before the events of the series, she was a loving partner to Kengo, and their relationship was serious enough that they were planning to marry. This background serves as the foundation for her entire role in the story, as her actions are consistently driven by the distorted remnants of her human emotions, specifically her obsessive love for Kengo.

As a character transformed into a Tekkaman by the alien Radam, Sword is an antagonist who serves the Radam cause and its leader, Tekkaman Omega, the entity her fiancé Kengo was turned into. Despite being an enemy of the protagonist, her personality is not purely evil or driven by a desire for conquest. Instead, she retains memories and feelings from her human life, which have been amplified and warped into a perverse, extreme devotion to Kengo. Her primary motivation is not loyalty to the Radam's goals but a singular, consuming need to protect the man she loves. She is willing to do anything to ensure his safety and well-being, a trait that makes her both dangerous and tragic.

Within the story, Teknoman Sword acts as a significant adversary to D-Boy, the Tekkaman Blade. Her role is that of a loyal lieutenant to Omega, often acting in his absence or on his behalf. At one point, she saves Tekkaman Evil from being destroyed by a Blasterized Blade, which allows her to become a leading force on the Radam side for a period. She orchestrates an attack using the Radam Mother ship in an attempt to eliminate Blade, and when that fails, she succeeds in destroying a critical space port on the Orbital Ring. Demonstrating a calculating and strategic mind, she actively works to strengthen her faction by tricking Shinya, the host of Tekkaman Evil, into undergoing the dangerous Blaster upgrade, knowing the severe side effects it would cause, in order to have a more powerful weapon against Blade.

Her most defining relationship is, of course, with Kengo Tekkaman Omega. Her love for him is the central pillar of her identity and the source of her strength and her ruthlessness. She also has a key, though antagonistic, relationship with D-Boy, viewing him as a primary threat to Kengo that must be eliminated. Her development is not one of redemption or change of heart, but rather a deepening of her tragic devotion. Her final actions are a testament to her unwavering obsession. After being critically wounded in a battle against the Space Knights, she endures her fatal injuries through sheer tenacity and love for Kengo. In her dying moments, she manages to severely damage the protagonists' ship, the Blue Earth, before ultimately succumbing to her wounds and burning up as she enters Earth's atmosphere, her last thoughts directed toward her lost love.

As a Tekkaman, Teknoman Sword possesses abilities typical of her kind, including the power of flight at incredible speeds and the capacity to survive and regenerate from damage. Contrary to her name, her primary weapon is a javelin-like tech lancer, which she wields in combat. She also possesses the Voltekka, an immensely powerful blast of anti-matter particles that serves as a Tekkaman's ultimate attack. For high-speed travel in its most aerodynamic form, she can utilize the Crest Interlude technique. Her abilities, while formidable, are portrayed as being within the average range for a Tekkaman, and she largely relies on her cunning and fierce determination to stand against her enemies.