A bit of more context of this scene:

This takes place in Demo's house, they're all having a sleepover there. By this time, Soldier has already ran away from home and stayed at Demo's place (to protect him from his father). In this period of time, Soldier doesn't want to go out that much because of paranoia and skepticism, so all of his friends gather at Demo's place and even have sleepovers. This warm yet funny scene has more depth considering this all takes place in a time where Engie puts his feelings aside to focus on Soldier's well-being. They grow closer, and in this sleepover Soldier wants to go out and feel the rain in the middle of the night, the only one awake was Engie and he goes with him (concerned). They have a sweet yet fun moment running in the rain and thus, this scene happens.