HOW TO UNLOCK THE TRUE ENDING
To unlock the True Ending we need to fulfil 5 requirements:
Clear the 1st 3 Sub Quests given by Asuna, Agil and Lisbeth during the Tutorial
Raise your affinity with Arfa-SyS, Kureha, Zeliska, Itsuki and Bazalt Joe to a minimum of 4 and 75%, can be equal or lower than that value but not lower
Raise your Affinity with Kirito, Asuna, Sinon, Leafa, Lisbeth, Silica, Yuuki, Argo, Klein, Agil, Premiere, Strea, Rain to a minimum of 2, can be equal or higher than that value but not lower
Clear Kirito Mode
Watch all the Cutscenes (Green Bubble on Map) with both your character and Kirito, the pillow scenes are 3 for each char so be careful about that
All this should be done before clearing the Main Mission: Mother Claudia, enter your home and you'll trigger a Cutscene with your Arfa-SyS where she'll give the Charm.
A couple more info: you have to clear Mother Claudia's mission at least once to unlock Kirito Mode so you're obliged to see at least 1 bad ending in order to unlock the True one.
For the Final Boss when you see him charging for his Dark Ball attack, be sure to stagger and block his charge or you'll die after that, hitting his weak spots on his chest or face might help you on that.
For a detailed walkthrough check the video below: