Ranger "Refugees" - [5th campaign, 1st mission]

Biara appears around day 30-40, depending on the difficulty level. If she doesn't show up after that time, you can safely restart the game; the scripts in this mission are clearly poorly written. I saw somewhere a bizarre suggestion to save at the very beginning and then skip 40 turns to make sure the script isn't lagging and Biara does appear. If she does, everything is fine; load the save you made on the first day and continue playing.