【That's how I am】 & 【That's who I am】

"That's how I am" sounds more like you're explaining your behavior. How you act in general.
I am a nice person. That's HOW I am.

 

 

 

"That's who I am" sounds more like you're explaining what kind of person you are.
e.g.)

I am a person who would help no matter the situation. That's WHO I am.