Who is A. R. Fullerton from Reminiscences of a Stock Operator and what is their importance?

A. R. Fullerton is the founder of the first Wall Street brokerage that Livingston trades in when he arrives in New York. Livingston loses a lot of money through trading at this firm while he is learning the ropes, and Fullerton gives him a loan so he can recoup his losses at the bucket houses and return to trading. Fullerton is kind and fatherly, which makes Livingston stay and lose even more money. Fullerton is well-connected and introduces Livingston to traders that have been around a while and know how to manipulate the markets. Livingston remains at Fullerton's office for his early career and learns many of his most important lessons there.