Get Relationship for the Author of the Latest Comment
I'm going to assume here that field_last_name is a field you added yourself to the account fields. I tested using just the user name, and got it to work fine using the following relationships (starting from a Node):


From the node to all comments: was called "Comments of the node using field: comment" here
And then from the comment to the author: just called "Author" here in the Comment category


Then I added the "Name" field (User category), and Bob was my proverbial uncle.