It could develop through shared experiences. Maybe they both get lost in a maze - like part of the forest together. During this time, they have to rely on each other's skills to find their way out. Lexa's tracking skills and Costia's knowledge of the stars for navigation combine. This experience cements their bond and they become closer friends, with Lexa starting to see Costia as a sister - like figure.
One possible plot could be that Kid Lexa and Costia meet in a forest. Lexa, being young, is curious about Costia who might be from a different clan. They start off a bit wary of each other but then find common ground in their love for nature. They could exchange stories about their respective clans' traditions regarding the forest, like how Lexa's clan uses certain plants for medicine and Costia's clan believes the forest has spirits that protect them.
Their relationship could start off as a mutual respect for each other's power. Gohan would be in awe of 18's android abilities, and 18 would recognize Gohan's potential as a Saiyan. As they spend more time together, they might start to become friends, sharing their thoughts and experiences.
Their relationship might start off very rocky. Snape, being his usual surly self, would be suspicious of the street kid Harry. But as time goes on and Harry shows his tenacity and good heart, Snape could start to respect him a bit more.
Lexa would likely become more protective and perhaps a bit softer in her approach. She has always been a strong leader, but now she has a family to think about. Clarke, on the other hand, would probably use her medical knowledge more to take care of the baby. Her nurturing side would be more prominent.
It might turn into a begrudging friendship. At first, they might not like each other. But as they face challenges together in the underground, like fighting off gangs or dealing with the harsh living conditions, they start to respect and care for each other in their own ways.
Their relationship could develop through a series of adventures. For example, they might have to stop a powerful villain together. In the process, Damian's impulsive nature could clash with Thomas' more methodical approach. However, over time, they realize that they can complement each other. Thomas could help Damian think more strategically, while Damian could inspire Thomas to be more bold and take risks in their fight against evil.
Their relationship could develop quickly. After meeting at the club, they might exchange phone numbers or some other means of contact. Then they would start to meet more often, like going for walks in the park or having coffee together. Their conversations would deepen, and they would learn more about each other's personalities and pasts.
Their relationship might start off rocky. Albus Severus Potter has grown up hearing different things about Snape, and Snape is not used to dealing with the son of Harry Potter. But as they spend time together, they could find common ground. For example, they both have a connection to the Potter family in different ways. Albus might start to admire Snape's intelligence and skill in potions, and Snape could see potential in Albus.
Well, if they meet differently, say through a magical accident that binds their fates in an unusual way. Their relationship development would be full of twists and turns. At first, they might be frustrated with the situation they are in because of this strange meeting. But as they try to figure out how to break the bond or at least understand it, they end up spending a lot of time together. This forced proximity makes them learn things about each other that they would have never known otherwise. Their relationship could start as reluctant cooperation but gradually turn into something more affectionate as they realize they have more in common than they thought.
In the fanfiction, their relationship could start with a bit of competition. Both are ambitious to be heroes. However, they soon realize that they can achieve more by working together. As kids, they might compete in small challenges like who can run faster or jump higher using their quirks (or in Izuku's case, his physical training). This healthy competition could fuel their growth and transform into a deep - rooted friendship that withstands all the difficulties they face later in their hero careers.