One way their relationship could develop is through a mutual understanding of their powers. If they start training together to combine their celestial magic in new ways, they'll have to communicate and rely on each other more. This would not only strengthen their magical bond but also their personal relationship. Another aspect could be that they face a situation where they have to make sacrifices for each other. Say, Lucy has to give up a chance at a powerful magical item to save Loke, or Loke has to put himself in great danger to protect Lucy's family. These kinds of selfless acts would take their relationship to a new level.
Their relationship could develop through mutual protection. For example, if Aries is in danger from an enemy, Loke could step in to defend her. This act of heroism might make Aries see Loke in a new light and start to trust him more. As a result, she could become more open with him, and they would start to build a deeper connection.
In a fanfiction, their relationship might develop if they start to open up more about their pasts. Lucy could share more about her family and how she came to be a celestial wizard. Natsu could talk about his search for Igneel. By sharing these deep parts of themselves, they would build a stronger emotional connection. Also, small moments like Natsu saving Lucy from a small danger, like a falling object in the guild hall, can add up and gradually develop their relationship.
In a fanfiction, their relationship might develop because of a mutual friend. Maybe a character from Fairy Tail is in danger and they both care about that person. So they work together to rescue the friend. During this rescue mission, they see different sides of each other. Angel might see Lucy's determination and kindness, and Lucy might see Angel's hidden strengths and softer side. This new understanding can lead to a closer relationship between them.
Lucy could give Aquarius more freedom outside of the key. Let her experience the human world more often. They could go on adventures that are not related to Fairy Tail's main quests, just the two of them. This would allow them to get to know each other on a more personal level. Aquarius could also teach Lucy more about celestial magic and its deeper secrets, which would bring them closer as they explore this knowledge together.
Their relationship could develop through shared secrets. For example, they might discover something about the guild's past that they decide to keep between themselves. This would create a special bond.
Their relationship could start as a simple acquaintance when they are assigned to the same mission. Then, as they face difficulties together, they gradually become friends. For example, Dan might save Lucy from a dangerous attack, and Lucy in turn helps Dan understand the workings of the Fairy Tail guild better.
Their relationship could develop through shared experiences. For instance, if they both get caught in a magical trap that only they can escape from by working together. They would have to rely on each other's skills, and this could lead to a newfound respect. Jackal might see Lucy's intelligence in figuring out the magic of the trap, and Lucy could see Jackal's tenacity.
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.
In this maid scenario, their relationship could develop as Natsu starts to see Lucy in a new light. He might notice her hard - working side more and become more protective of her. For example, if other guild members tease her about her maid outfit, Natsu could get angry and tell them off.
In the shower scenario, their relationship could develop through shared vulnerability. For example, if there's a problem like the water suddenly turning cold, they might huddle together for warmth. This physical closeness could make them more aware of their feelings for each other.