I cannot see the exact amount of heath my Mystic Ally has. I tried having it take damage to see if it would show a health bar (similar to monster's health), but this did nothing. How do I know how much health he has? Does it ever increase?
The Mystic Ally most likely has the exact same hitpoints and armor as you have.
I tested this by getting out of my armor and having a single enemy attack me, I was a sliver of health from death after 8 hits. I then cast the Mystic ally and it was nearly dead after 8 hits.
I'm not counting in the hits I dodged, and in my test the Ally did not dodge a single hit.