In this memoir, for quasi-split classical groups over p p -adic fields, we determine the L L -packets consisting of simple supercuspidal representations and their corresponding L L -parameters, under the assumption that p p is not equal to 2 2 . The key is an explicit computation of characters of simple supercuspidal representations and the endoscopic character relation, which is a characterization of the local Langlands correspondence for quasi-split classical groups.