There is defined role of a Business Analyst in Agile methodologies specially SCRUM. So, it is subjective to practitioners interpretation. However, the roles and responsibilities of an Agile Business Analyst are very obvious. I have written an article on the topic:
Who is an AGILE Business Analyst? Do Agile Projects need a Business Analyst?
This article deals with the topic in details. In case, you are interested to know little more, watch our webinar (approximately 1.5 hrs) on the role of the business analyst in Agile projects