"You are a beautiful person."
Translation:Olet kaunis ihminen.
That sounds strange in Finnish. Word for word translations don't always work.
Henkilö is a common word, but something you use when you want to be impersonal and keep a great distance, in a good way: when you talk about how many people do this, when you want to retain anonymity for a person, etc. It's very formal in spirit, although it can also be used in less formal contexts.
Ihminen, on the other hand, is a human being, with the focus on that. So you will usually see ihmisiä in news reporting, for example, only if there is a reason to bring "a human touch" to the story.