You have to select the correct choice with the Arlish ambassador in the welcome feast. I was also having the same issue with getting Avalie as my ally as I selected the option "The final 2 weeks are more important than the previous 5". During the nego, you will have 2 choices to select during the ambassador showdown - if you only got to support and give (un)due credit to Cato during the week 7 nego, you will not get the legendary ending.