Q:

How to refer to someone who has depression(A noun for someone who has depression)?

What do we call a person who is suffering from depression?
Usually I hear "X has depression" but can I say *"X is a 'depressive'"? 
I have heard the word depressive used as a noun before; but I'm not sure if it is right. 
We do use diabetic to describe someone who has diabetes, so is there a parallel word there?

A:

I have heard the word depressive used as a noun before but I'm not sure it is right.

Yes, it is right (especially in the plural). For example:

Depressed mood is often associated with a variety of physiological maladies, and many depressives are burdened with an assortment of ailments ranging from migraine, colitis, and allergies, to endocrine disorders and menstrual disturbances. [link]

