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.
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.
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.
Their relationship could start with a simple truce. After a fight, they both realize that they are evenly matched and instead of constantly battling, they decide to call a temporary halt. This could lead to them having conversations and slowly understanding each other.
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.
In fanfiction, they could have a sort of brotherly relationship. Boruto might see Kid Naruto as a younger version of his father, but also as an individual with his own charm. They would play pranks on people together, and when it comes to serious ninja business, they would have each other's backs. Boruto would protect Kid Naruto from some of the tougher enemies he might not be able to handle yet, and Kid Naruto would keep Boruto's spirits high with his positive attitude.
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 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.