There are three levels of formality in Hindi in the second person- तू ,तुम and आप. तू conjugates like it is singular and the other two conjugate like they are plural (though all three can be used even when you are talking to a single person).

So, the three different ways of asking a male person "How are you" are:
तू कैसा है - to be used only for close male friends or family
तुम कैसे हो - to be used for males younger or the same age as you or to convey intimacy
आप कैसे हैं - to be used for men older than you or when you are being formal/respectful

For females, both the singular and plural use कैसी:
तू कैसी है - to be used only for close female friends or family
तुम कैसी हो -to be used for females younger or the same age as you or to convey intimacy
आप कैसी हैं -to be used for women older than you or when you are being formal/respectful