Cloud Murderer

An unflappably optimistic and sunny person; a term generally used by pessimists and nihilists who've had their dark mood spoiled by a random act of kindness.

Person A: "Woah, what happened to you? You're smiling!"
Person B: "I know... this fucking cloud murderer came out of nowhere, told me they liked my jacket, handed me a flower, and told me to have a nice day! I didn't know what to do, so I smiled back and said thank you..."