Maybe the reader is a noble who has a secret fascination with death. They meet Undertaker by chance and he's intrigued by their unique view. The two get involved in a mystery that involves the afterlife and the dark secrets of the noble society in the Black Butler universe. They rely on each other's strengths to solve it.
The reader could be a reaper who has lost their memories. Undertaker finds the reader and decides to help them regain their memories. Along the way, they develop feelings for each other. But this is complicated by the reaper society's rules and the chaos of the Black Butler world.
One idea could be that the reader is a new undertaker's apprentice. They start off being clumsy and Undertaker finds it both amusing and endearing. As time passes, a deep bond forms between them, and they face various supernatural challenges together in the world of Black Butler.
One interesting plot could be Undertaker's past being explored in more detail. Maybe he had a relationship with one of the other main characters' ancestors that comes to light and affects the present story. For example, if he had a connection with Ciel's family long ago.
One idea could be that the Undertaker is secretly protecting the main character of Black Butler out of love. He uses his unique skills and knowledge to keep them safe from various threats in the dark underworld of the story.
One plot idea could be that Vincent, before his death, made a secret pact with Undertaker that is only revealed years later. For example, it could be related to the future of the Phantomhive family and the underworld they are involved in. Another could be a story where Undertaker has memories of Vincent that he has never shared, and these memories start to surface when he meets Ciel, causing him to act in unexpected ways towards Ciel. Maybe there's a plot where Vincent's spirit is trapped in a limbo-like state, and only Undertaker can free him, but it comes at a great cost to the balance of the afterlife world in the Black Butler universe.
One plot idea could be the reader being a new servant in the Phantomhive manor. Ciel is initially suspicious but as time passes, he starts to trust and rely on the reader in a unique way. Maybe the reader has a special skill that helps Ciel in his investigations or in dealing with the other nobles.
One common theme is mystery. Undertaker is a very mysterious character, and this often seeps into the fanfiction. The reader is usually drawn into his world of secrets. Another theme is the contrast between the darkness associated with Undertaker and the possible light or innocence of the reader - character.
One aspect to consider in 'Black Butler Undertaker x Reader' fanfiction is the unique relationship between Undertaker and the reader - insert character. Undertaker has a very mysterious and alluring personality in the Black Butler universe. His interactions with the reader - character can be explored in various ways, such as through shared secrets or his interest in the reader's unusual qualities. Writers often play with his character traits like his love for death - related things and his rather eccentric behavior to create engaging storylines that don't rely on inappropriate content.
I really like 'Undertaker's Affection'. In this fanfic, the relationship between the reader and Undertaker grows in a very natural way. The story has some really sweet moments, like when Undertaker shows his softer side towards the reader. It also weaves in elements from the original Black Butler story, which is a plus.
A good 'Black Butler Undertaker x Reader Fanfic' should first and foremost capture Undertaker's character accurately. His mysterious and somewhat macabre nature needs to be portrayed well. For example, if he's suddenly too out - of - character and overly nice all the time, it doesn't feel right.
One interesting 'Black Butler Undertaker fanfiction' story could be about Undertaker's past and his connection to the Phantomhive family in more depth. Maybe it reveals secrets that were only hinted at in the original series.
In the fanfiction, have them face challenges together. For example, they could be investigating a series of strange deaths in the city. During this time, they rely on each other's skills. The reader might be good at gathering information from the living, while Undertaker has his knowledge of the supernatural. This shared experience and dependence on one another can make their relationship seem more real and deep in the context of Black Butler.