We say "¿Quién eres tú?" for "who are you?" in Spanish ok.
So why can't we say "¿Cómo eres tú?" for "how are you?" in Spanish?
"Eres tú" means "are you" so why is "Cómo eres tú" incorrect for "how are you"?
The thing is that translating the verb be to spanish, it has to be done with two verbs with different meanings: ser and estar. And they can't be used interchangeably. If you think about it, it's a little simpler for us speaking spanish because como estás tú? and como eres tú? are two completely valid questions but with completely different meanings. One could be answered bien, mal, contento, molesto, aburrido, dormido, etc... the other one could be answered: inteligente, alto, bajo, gordo, flaco, parlanchín, cándido, maléfico, etc.
So, if what you mean is to greet someone, you mean como estás tú?. If you are inquiring about how the person would define him/herself, you would ask como eres tú?.