There are indeed some 'Naruto Lucy sister' fanfictions out there. However, the quality can vary greatly. Some are written by very talented authors who manage to create engaging storylines that are in - line with the Naruto world while adding the unique element of Lucy as a sister figure. They might develop her character in a way that she has her own goals and conflicts, and how those interact with Naruto's story. But there are also some that might not be as well - written, so it's important to read reviews or summaries before diving in.
One possible plot could be that Lucy is a long - lost sister of Naruto. She has been living in a different village all this time and has unique ninja abilities. When they finally meet, they have to work together to face a common enemy that threatens both their villages.
They could have an adventure where they have to stop an evil wizard who is stealing chakra and magic energy. Naruto uses his ninja skills and Lucy uses her celestial spirit magic to track down the wizard and defeat him.
One way to develop her character is by making her have a special goal. Let's say she wants to become the most powerful mage in the world, not for glory, but to protect Lucy. This goal drives her actions and decisions throughout the fanfiction. She trains hard, seeks out powerful opponents to test herself, and along the way, she has to face her own weaknesses and fears. Through these experiences, her character becomes more complex and well - rounded.
One idea could be that Lucy's little sister has a special magical ability that is different from Lucy's. For example, while Lucy uses celestial spirit magic, her sister could have nature - based magic. In the fanfiction, she might start out as a bit of an outcast in the guild because her powers are so different, but then she gradually earns everyone's respect through her determination and kind heart.
They could go on a quest to find a powerful magical artifact. Naruto's combat skills and Lucy's knowledge of magic could be useful in navigating dangerous dungeons and fighting off enemies who also want the artifact.
In the Naruto series, Naruto doesn't have a biological sister in the original story. So in this fanfiction, the author probably creates a new character to be his sister. It could be an original character with unique abilities and a complex backstory that leads to the situation where Naruto dies by her hand.
Naruto would probably be very enthusiastic and friendly towards Lucy right away. He might be curious about her celestial spirit keys and ask a lot of questions. Lucy, on the other hand, might be a bit more cautious at first but would be drawn to Naruto's positive energy.
A Naruto and Lucy Heartfilia fanfiction crossover could be really interesting. One idea could be that they meet in a magical realm that combines the elements of their two worlds. Naruto's ninja skills and Lucy's celestial spirit magic could be used to fight a common enemy. For example, they might team up against a powerful dark wizard who is trying to take over both the Fairy Tail world and the Hidden Leaf Village. This would lead to a lot of action - packed scenes where they learn from each other's fighting styles and develop a friendship along the way.
A fanfiction involving Naruto and Lucy Heartfilia might explore their different personalities. Naruto is often hot - headed but has a big heart, while Lucy is more level - headed and empathetic. When combined, they could make a great team. For instance, in a story where they are trying to rescue kidnapped friends from an evil organization. Naruto would be the first to rush in, and Lucy would be right behind him, using her strategic thinking to support his actions.