Harry - Draco
1. Years after the war is over, Harry find his old trunk with a jean that never was his. He remembers (or asks Draco to remind him) how this piece of clothing ended up in his school trunk.
2. Draco has got the flu and Harry ran into trouble. Both are stuck in the Healing Ward of the school.
3. "That's a mobile phone". Harry tries to explain Muggle things to Draco (post-Hogwart). Friendship or established relationship.