I think Sir Ian McKellen would be a great choice. He has that air of elegance and mystery about him. His acting skills are top - notch and he could bring a lot of depth to the role of the butler.
Maybe Alan Rickman could have been a good fit too. He was so good at playing complex characters. His voice and presence would have made the butler a very interesting character. He could have added a touch of sarcasm and dry humor to the role.
Another option could be Timothy Spall. He has a great ability to play characters with different personalities. As the butler, he could be both kind and firm, always looking out for Harry's best interests while also keeping him in line when necessary.
Dobby could be a great butler. He was always eager to serve Harry and was very loyal. His knowledge of the Malfoy manor and other wizarding households could be useful. And his unique brand of magic, like his ability to apparate in and out of places where other wizards couldn't, would add an interesting dynamic to the story.
A character like Severus Snape could be an interesting choice. He already has that mysterious and somewhat dark aura about him. Transforming him into a demon butler would add an extra layer of complexity to his character, and his relationship with Harry could be re - imagined in a whole new way.
Another option could be Michael Caine. He has that classic British charm and elegance that would be perfect for a butler role. He could bring a sense of humor and warmth to the character, while still being able to handle the more serious and magical aspects of the story.
Dobby could be a great character as a butler in fanfiction. He already has a strong loyalty to Harry and his self - sacrificing nature would make him an excellent butler. He could use his house - elf magic to keep Harry's living quarters clean and protected.
The butler could be a protector. He might use his skills, like combat skills or knowledge of ninjutsu, to keep Naruto safe from various threats, whether it's rogue ninjas or political schemes.
Sebastian and an angel could make for an interesting pairing in a mate - based fanfiction. The whole concept of a demon and an angel being mates goes against the natural order in the Black Butler universe. It would create a story full of moral dilemmas, as they try to reconcile their different natures and the duties they have towards their respective realms.
Joker might be a key figure in a mystery or crime plot. His knowledge of the seedy underbelly of the circus world could translate into knowing about illegal activities in the modern underworld. His ability to deceive with his performances could also be used to his advantage when dealing with criminals or those trying to uncover his secrets.
One idea could be that the butler has a secret magical ability that he uses to protect Harry from hidden threats in the wizarding world. Maybe he can turn invisible at will and spy on Harry's enemies.
One possible plot could be that the butler is actually a wizard in disguise who has been sent to protect Harry. He could use his knowledge of the wizarding and Muggle worlds to keep Harry safe from both magical threats like Death Eaters and Muggle dangers. For example, he could use his connections in the Muggle world to find safe houses for Harry when he needs to hide from Voldemort.
Sakura might be initially suspicious. She is a smart girl and would wonder about the butler's intentions. She might keep a close eye on him to make sure he doesn't harm Naruto in any way.