First, create situations where they are forced to rely on each other. Maybe they are trapped in a strange dimension and need to combine their powers to escape. As they do this, they start to respect each other's abilities. Then, introduce a third party that tries to drive them apart. This could be another Espada who is jealous of Halibel's attention to the OC. Overcoming this obstacle would strengthen their bond. Also, show the OC learning about Halibel's culture in Hueco Mundo and Halibel learning about the OC's unique abilities and how they can be useful in different situations.
To develop their relationship, start with small interactions. For example, the OC could save Halibel from a surprise attack by other Hollows. This would make Halibel curious about the OC. Then, have them exchange stories about their lives. Halibel could talk about her rise to become an Espada and the OC could share their origin story. Another way is to give them a common goal, like protecting a group of innocent Hollows from a cruel Arrancar. Working together towards this goal would bring them closer.
Well, for a Bleach Halibel x OC fanfiction, a plot could revolve around the OC being a time - traveler. The OC travels back in time and meets a younger Halibel before she became an Espada. This could change the course of Halibel's life and their relationship. Or, the OC could be a musician in the human world whose music can reach the spiritual realms. Halibel hears this music and is drawn to the human world to find the source, which is the OC. Also, the OC could have a special ability to communicate with water, just like Halibel has an affinity with water - based attacks. They could bond over this shared connection.
In a post - war scenario, their relationship could develop into a mutual respect kind of thing. They've both seen the horrors of war, and they understand each other's strength. They might start to meet occasionally to exchange stories and advice on how to keep their respective groups safe.
Their powers could be combined in a way where Ichigo's Zanpakuto spirit, Zangetsu, and Halibel's water - based powers merge. For example, Ichigo could gain the ability to use water - enhanced slashes that have the cutting power of his normal attacks along with the fluidity of Halibel's water attacks.
To develop their relationship, start with misunderstandings. Let the OC misinterpret Toyohisa's brash behavior at first. Then, as they get to know each other better, they start to see the real person behind the facade. Have them have deep conversations about their pasts, their hopes, and fears. Also, physical contact can play a role. A simple touch on the arm during a tense moment can be the start of something more. And don't forget about conflict. Maybe the OC's pacifist views clash with Toyohisa's warrior ways, but they learn to respect each other's differences.
To develop their relationship, start with small interactions. Have the OC offer to help Lucy with her celestial spirit summoning, like finding a rare ingredient for a spirit - summoning spell. Then, they could have some shared experiences, such as getting trapped in a cave during a mission. This forces them to rely on each other, talk, and get to know one another better. Another way is to give them common goals. For example, they both want to defeat a certain dark mage who has been causing trouble in the area. Working towards this goal together will bring them closer.
Communication is key. They could start by sharing their pasts, like Ichigo talking about his journey to become a Soul Reaper and Halibel sharing about her life in Hueco Mundo. This would help them understand each other better.
Ichigo would develop a more patient side. Halibel's different way of thinking and her more reserved nature would force Ichigo to slow down and really think about his actions. He would also become more protective, not just of Halibel but also of the new relationship they are building. Halibel would start to open up more. She was always a bit of an enigma in the original story. But in this fanfiction, being married to Ichigo would make her more expressive. She would start to share her thoughts and feelings more freely, and her character would become more complex as she tries to balance her old self with the new self that is influenced by Ichigo and his world.
One common theme is power. Since Halibel is a powerful character, many fanfics explore her power further, like how she gains more power or how she uses it in different situations.
You can start with them having a bit of a rocky start. Maybe Alex is too brash for the OC's liking at first. But as they face challenges together in Jumanji, they start to see the good in each other. For example, Alex's courage impresses the OC during a fight with a game - created monster, and the OC's intelligence helps them solve a difficult puzzle, earning Alex's respect.
It could start with Ironhide showing simple kindness to the OC. Just small acts like making sure they have food or a safe place to stay. This can gradually build into a deeper emotional connection.