Anastacius originally has short, straight blonde hair neatly parted to the right in a fringe falling over his eyes, with the left side slicked back. Like the rest of his relatives, he inherited the imperial jewel-blue eyes. Since his return in the current timeline, Anastacius has been using magic to disguise himself from those who may recognize him, transforming the color of his hair and eyes to black. Occasionally, the magic in his eyes will slightly fade and subtly reveal their original jewel-blue color. In the middle of his chest is a large tissue scar, a souvenir from his fatal encounter with Claude. Anastacius wears a variety of nobility outfits throughout the series, typically opting for dark colors such as grey and black. As a child, Anastacius is depicted as an empathetic, kind-hearted person, denoted by his sympathy towards Claude’s plight during their childhood. He undergoes a radical change as he grew older, eventually becoming a duplicitous, callous and sarcastic individual who regarded his younger, talented brother with open disdain. Since returning and being possessed by Aeternitas, Anastacius alters his behavior depending on the person he interacts with. Towards Jennette, Anastacius exhibits a warm, amiable demeanour with an air of mysterious allure to him. To Roger, Anastacius expresses himself using a very blunt, and occasionally insulting and vulgar, manner of speaking. He possesses a dark sense of humour, and typically takes on a nonchalant and laid-back approach to things. Athanasia remains skeptical of his true personality, recalling his cold expression during her secret outing with Jennette and how suspicious his overly polite nature always seems. However, Anastacius's temper occasionally breaks through his self-control, causing him to say things that he later regrets. In stark contrast to Jennette, who believes a family should have unconditional love for each other, Anastacius perceives familial bonds in a negative manner due to his difficult upbringing. However, Anastacius is capable of taking advantage of his own daughter's naïveté to further progress his plans. He demonstrates his ability to think tactically when he curses the blue ribbon gifted to Athanasia, which serves as a catalyst for Claude's memory loss. Despite his manipulative tendencies, Anastacius refrained at the last moment from cursing his own daughter after recalling her sincere joy at reuniting with those whom she believed were her family. He even faltered at the last minute at the golden opportunity to easily eliminate Claude’s mother, instead turning his back on her under the claim she was already dying.