I'm not sure specifically as there could be many novels with that title or concept. Generally, 'patient zero' in a horror context often refers to the first infected or the origin of a terrifying epidemic or curse in the story. It might involve the protagonist trying to figure out how to stop the spread that originated from this patient zero while facing various horrors along the way.
I don't know the specific main characters in 'patient zero horror novel' as I haven't read it. But it could have a scientist who is trying to find a cure for the horror that patient zero has unleashed. There might also be a survivor who has some special skills or knowledge that helps them in the fight against the spreading horror.
Character development. Writers can create OCs with their own growth arcs that interact with the existing characters. It could be an OC familiar that starts off as a weakling but grows into a powerful ally, or an OC mage who has to learn the ways of the Tristain academy. All these development paths add depth to the fanfiction. Also, the relationships formed between the OCs and the original characters, like a new love interest for Saito or a rival for Louise, can create exciting new storylines. This allows for a lot of creativity within the framework of the 'Familiar of Zero' universe.
First, give the OC a unique backstory. For example, if she grew up in a war - torn village, it could shape her personality as someone who is strong - willed and determined to prevent more destruction. Also, give her specific skills like being able to communicate with animals, which can add an interesting dynamic to her interactions with Sub Zero.
One great Fate Zero OC Assassin fanfiction is 'The Hidden Blade'. It delves deep into the backstory of the OC Assassin, creating a rich and complex character. The plot weaves in and out of the main events of Fate Zero, adding new dimensions to the story. Another interesting one is 'Shadow's Purpose'. It focuses on the Assassin's unique abilities and how they interact with the other Servants in the Holy Grail War. And 'The Assassin's Vow' is also worth checking out, which shows the Assassin's moral code and how it is tested during the war.
You could create an OC who is a master of illusions. This character could use their powers to deceive both enemies and allies, and their backstory could involve being an outcast from their own illusionist clan because they refused to use their powers for evil. In the Re:Zero world, they could help Subaru by creating diversions during battles or by using illusions to uncover hidden truths.
One thing that makes Fate Zero OC Assassin fanfiction stand out is the opportunity to create a completely new character within the rich and detailed world of Fate Zero. The Assassin in these fanfictions can be designed to fit into the existing lore while also bringing something new. They can have a backstory that ties into the history of the Grail War in unexpected ways. For example, an OC Assassin could be a remnant of an ancient cult that has a secret agenda related to the Grail itself. This kind of creativity makes the fanfiction very appealing.
You could include a wise old inmate who has been in the prison for a long time and has a lot of knowledge about its secrets. This character could help the oc and Prisoner Zero in their plans.
To integrate a male OC well, he should have a connection to the existing elements of the story. For example, if he has a past with the Void magic that is so important in the Familiar of Zero world. Maybe his family was involved in some way with the secrets of the Void. Or he could be related to one of the minor characters that we haven't seen much of in the original story. This would make his presence feel more natural.
In Zero Tails fanfiction, a great OC host could be a character with unique magical abilities. For example, a character who can communicate with spirits might be an interesting host. This allows for various plot developments as they can gain insights from the spirit world and use that knowledge to either help or hinder the Zero Tails' actions.