Does a greeting require a comma between Hi or Hello and the recipient's name?

No -

Hello Ralph,

I was wondering if we could discuss ....

Yes, there should be a comma between "Hi" or "Hello" and the person's name because it is a "direct address" to the reader and not about the reader. Most people fail to use this formal rule of etiquette and writing because they failed to be educated to do so. In emails, due to their general informal nature, use of the comma between the greeting and person's name is not required, but is nonetheless appreciated as the formal standard rule of punctuation. The comma should always be used between the greeting and person's name, however in a business letter. * Never use a comma between "Dear" and the reader's name.