Well, that's a bit tricky. The ending of the manga doesn't explicitly reveal who Anzu ends up with. It sort of keeps you guessing and leaves it up to your imagination.
I found some recommendations for novels similar to Romance Killer. Here are the related novels I found for you:
1. Book of Troubled Times by Ji Cha
2. "Spirit Realm Walker" Author: Paper Selling Little Langjun
3. Tang's Table Author: Jie Yu
4. King of the Familiar by Light Spring
5. Fairy Comes Author: Blue Dream
These novels may have some similarities with Romance Killer. I hope you will like them.
Maybe a heroic character like a detective or a super - powered being within the fanfiction will come to Anzu's rescue. This character could have been tracking Prisoner 777 all along and finally catches up to him when he kidnaps Anzu.
Anzu could use her wits. If she can find a moment when Bakura is distracted, she might be able to run away. Maybe she could create a diversion, like knocking over something noisy to draw his attention and then make a break for it.
Anzu would probably be scared at first, but then she'd likely start to think of ways to escape. She's a brave character, so she wouldn't just sit there and accept her fate.
To write this fanfiction well, first, you need to understand the characters of Marik and Anzu. Research their personalities, backstories, and how they interact in the original Yu - Gi - Oh series. Then, when Marik calls Anzu 'little Anzu', make sure to show the emotions behind it. Is it out of kindness, or is there some ulterior motive? Also, build a good plot around this moment. It could be a simple slice - of - life story or an epic adventure, but it should revolve around their relationship.
Maybe it's a fanfiction set in a more domestic setting. Marik has to take care of Anzu for some reason and starts using the name 'little Anzu'. The story could show Marik's growth as he learns to be more responsible and caring. Anzu, on the other hand, might initially resist this new dynamic but eventually come to appreciate Marik's efforts. It could also include some cute moments like Marik making Anzu her favorite food and them having heart - to - heart conversations.