a ray of sunshine
1. If you describe someone as a ray of sunshine, you mean that they are always happy and pleasant. Neighbours described the 4-year-old girl as `a ray of sunshine — a cheerful little girl who brightened up all our lives'. Note: You often hear people say that someone is like a ray of sunshine. Kim is like a ray of sunshine, a wonderful and beautiful girl who has completely changed my life. Note: This expression is sometimes used ironically to describe a person who is always miserable and negative.
2. If you describe something as a ray of sunshine, you mean it brings happiness, often during a sad or hopeless time. Emily Jones's first birthday next week will be a ray of sunshine in the darkness of the coming days. Note: You often hear people say that something is like a ray of sunshine. The news was like a ray of sunshine in the winter's gloom. Note: You can also say that something brings a ray of sunshine to someone or something. The Oscars bring a ray of sunshine to our dark February days.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012