It is the first song of the series. The song talks about a dying man who lived in a city and that, after seeing an unknown, falls for her. We can say it is a platonic love. However for some reason he has to get away from his beloved, and without time to explain, he leaves a message. Not knowing if she will read (since it never spoke) he hopes that one day she finds this message and read.
The second song of the series and the sequel of Rose + Thorn. The story now has a flashback about the past of the dying man, there is shown a new fact: he contracted a disease which he had once before ("I've caught the virus again."). He possibly believed it had healed but this was not what happened. Perhaps fearing his death, he reflects his sad state while watching the rain fall.
The third song of the series and sequel of Anonymous. The song tells the story from the point of view of the unknown. Though she never ask his name, she always saw the dying man in the same place every day waiting for someone who seemed to not come. After returning to her home, she finds a note on her door and discovers that the person he always hoped to see was her. His excitement was great to discover his love for her was so about to fall to his knees. In the following days she tries to look for him, but she cannot find him. She promised herself that one day they will meet again and this time, she'll be by his side.
The fourth song of the series. Comes another new person in history: the girlfriend dying. Although very sad, she tells her relationship with him (evidently showing that wanted to be more than her friend) and regrets that he had this feeling for another girl and not for her. In some stretches reports saying that the chain that bound them were already rusty. So seeing that will never have his unrequited love, she will give the key to free him and that her only wish is that the other make him happy.
The final song of the series. The music would be an account of his dying friend before leaving, and comes as a fact before "Witness". He says that from the beginning would be alone (since the unknown did not answer your message). Seeing that his female friend was crying for his departure, saying that he dismisses all the moments spent there was never sad and that would now be leaving without the love he had.