Susan "Mandark" Astronomanov is the yandere to Dee Dee in Dexter's Laboratory.
Mandark is a tall boy who has a black bowl-cut hairstyle that lays around the top of his head like a helmet. It has a white shiny glare on it and an M shaped opening at the bottom. This hairstyle was passed down to him by his father Windbear which was also passed down to his little sister Lalavava. Mandark has a head that is big at the top and gets skinnier at the bottom near his neck. He has nerdy glasses which look exactly like Dexter's and he wears a white dress shirt, a yellow tie with black stripes, blue shorts, and brown shoes.
Mandark is an evil boy genius who is bent on the destruction of Dexter's Laboratory and ruling the world. He is an incredibly intelligent individual who, when first introduced, was leagues ahead of Dexter in terms of knowledge and accomplishments, but after having his lab and heart destroyed by Dexter and Dee Dee, his scientific skills became far less impressive than Dexter's, likely due to the mental trauma of seeing his greatest works completely destroyed and having his first love be used against him. Ever since, Mandark has always been one step behind Dexter and has done all he can to get the better of his rival and prove once and for all that he is the superior boy genius, but every time he tries to, he always ends up failing horribly. After Mandark's laboratory was destroyed, he eventually built a new one to resemble the old, but eventually he remodeled his lab and gave it a darker and more gothic-look, with many of the machines being replaced with bio-mechanical equipment; apparently meant to represent Mandark's growing evil nature, which was a result of his ever growing hatred for Dexter and his constant failures. His dark desires and evil ego continued to grow throughout his life, and by the time he reached adulthood, he became a full-blown supervillain who desired the complete take over and enslavement of the planet. It is implied that Mandark's dark nature is due to living a lonely life, as mentioned in "Mandarker", where he states that he has no friends or hobbies and the only thing he had in life was his lab and experiments. Eventually though, Mandark would gain an interest in surfing after "Sun, Surf, and Science" and gained his first real friend in the form of the Surfer Boy. In seasons 3 and 4, Mandark's motives and personality were drastically changed, with the story implying that he had always been evil even since his birth and always had the gothic motif he had in said seasons.
However, there is another side to Mandark's personality, which is his intense love for Dee Dee. Ever since he first laid eyes on her, Mandark has been completely enamored with her to the point of obsession and whenever he is not focusing on his evil plans or hatred towards Dexter, he only thinks of his beloved Dee Dee and dreams of what kind of relationship they would have. Despite this, Dee Dee has shown no interest in Mandark and at times seems to outright hate him or look down on him, and barely ever shows any non-hostile feelings towards him, but regardless, Mandark's love for her remained unchanged throughout the series and he continued to love her unconditionally. In fact, even when Mandark was completely corrupted and became the evil and heartless Overlord Mandark in the future, the one thing that remained unchanged about him was his eternal love for Dee Dee which is without a doubt a permanent trait of his personality that would stay with him even in his later years. It was this love that would eventually lead to his own defeat in the future. Regardless, Mandark continues to love Dee Dee above all else; caring more about her well-being than even Dexter at times, with Mandark continuing to love her even when she destroyed his future empire while Dexter instead wanted her destroyed for taking the glory from him.