Isolation is also a theme. Alucard is often alone in his existence, being one of the most powerful vampires. His long life and the things he has seen and done set him apart from others. This is shown through his interactions, or lack thereof, with other characters in the novels. He is a solitary figure, yet tied to the events around him.
One common theme is power. Alucard is a very powerful character, so fanfictions often explore how he uses his power, how he gains more power, or how he struggles with the consequences of his power. Another theme is redemption. Given his often dark and violent past, many stories deal with Alucard seeking some form of redemption.
One common theme is the exploration of her power in a different gender context. For example, how she might use her vampiric abilities differently as a female. Another theme could be relationships, like romantic relationships with other characters in the Hellsing universe, which might be portrayed differently compared to when Alucard is male.
In many Alucard and Maria fanfics, the theme of growth is prominent. They both grow as characters as they interact. Maria may grow in her skills and understanding of the world, while Alucard may grow in his ability to connect with others. Additionally, the theme of facing danger together is quite common, as they often battle against various foes in these stories.
One frequent theme is the idea of identity. Harry, with his own identity as a wizard and the Boy Who Lived, has to grapple with Alucard's identity as a powerful vampire. Also, the theme of friendship or rivalry might be present. They could start as rivals due to their different natures but end up as friends who respect each other's abilities.
One common theme is the exploration of their different natures. Since Bella is often portrayed as a more innocent or human - like character, and Alucard is a powerful vampire, there's a contrast. Another theme could be power dynamics, like how Bella might be influenced by Alucard's power or how she might change in his presence.
Power struggles are a common theme. Since Alucard is very powerful, when he crosses over into other universes, there are often clashes with powerful characters there. Another theme is the exploration of his identity. In different crossovers, his vampire nature and his moral compass are re - examined.
One common theme is loyalty. Seras is often portrayed as being extremely loyal to Alucard, and this loyalty is tested and strengthened throughout many fanfics. For example, in some stories, she has to choose between following Alucard's orders or her own moral compass, but her loyalty to him never wavers completely.
A common theme is the power exchange. Naruto might learn some of Alucard's dark powers, or vice versa. Another theme is the cultural clash between the vampire world of Alucard and the ninja world of Naruto.
Power struggle is a common theme. Since both Alucard and Sebastian are powerful in their own right, fanfics often explore who would be more powerful in different situations. Another theme could be loyalty, as they are both loyal to their respective masters in their original stories, and fanfics might play with that loyalty in the context of their encounter.
A common theme is the battle of ideologies. Anderson, being a devout Catholic and Alucard, a vampire, they have very different outlooks on life and death. Fanfictions often explore how these clashing beliefs interact when they face each other.