Wordorigins.org

In the latter half of the 19th century, the word huckleberry came to denote a fellow or man in American slang. It was usually used as term of affection to a friend, but could denote a foolish or incompetent person as well. Sometimes it was used to mean a person particularly well-suited to a job or task, often in phrases like I’m your huckleberry.

The sense meaning a general person or fellow dates to 1868. From the New England Base Ballist of 3 September of that year:

Now then, my huckleberry, look sharp! you’re wrong!