In a fanfiction, you could have Artemis and Icicle Jr start off as enemies. But then, introduce a common enemy that is more powerful and dangerous than either of them. As they fight this new foe, they see the good in each other. Artemis might notice Icicle Jr's sense of honor in battle, and Icicle Jr could see Artemis's determination. This would gradually change their relationship from hostility to grudging respect and then, perhaps, friendship or something more. Well, in a more complex way, you could also add some emotional elements. Maybe Icicle Jr has some insecurities about his place in the world of villains and heroes. Artemis, having her own issues with identity in the Young Justice team, can relate to him on a deeper level. They could have long conversations during lulls in the action, where they open up about their fears and hopes. This would not only develop their relationship but also add depth to their characters. For instance, they could be hiding in a cave waiting for backup, and that's when they start really talking. This would be a great opportunity for the writer to explore their personalities and how they can grow closer.
Another way to develop their relationship is by having them work on a long - term project. Let's say they are part of a group tasked with rebuilding a destroyed city. Artemis is good at strategic planning and Icicle Jr has his ice - based powers that can be used for construction. As they work side by side day after day, they start to see each other in a new light. They might start off arguing about the best way to do things but end up compromising and learning from each other.
Their relationship could be developed through shared experiences. For example, if they get trapped in a dangerous situation together, like in an abandoned ice fortress. They have to rely on each other to survive, which would break down some of the barriers between them.