This is story related in your third playthrough and cannot be missed. Depending on your choices, though, it will unlock at different points during your third playthrough. Following chapter 11, there will be several interludes where Pods 042 and 153 interact with one another. At the end of their interaction, you will be given a choice to support 9S or A2. If you choose A2, following a cut scene, you will gain control of A2 for the first time and the achievement will unlock.
Recommendation: When you regain control of 9S towards the end of the chapter 11 main story mission,
Loss, make a
save at the City Ruins: Near the Tower access point. Do not overwrite this save file until after you get this achievement.
The first available opportunity to unlock the achievement is immediately following the
Loss main story mission. Select A2 the first chance you get and the achievement will unlock following the cut scene. You do not have to select A2 at first. There are some advantages to play first as 9S. Only he can get the materials to fully upgrade the pods. Conversely, there are some advantages to play as A2. If you want to do some leveling up, she is a powerhouse who can level up more quickly than 9S. So, the decision of whom to play first is a matter of personal preference. Eventually, you will play as A2 and the achievement will unlock when you first do so following the introductory cut scene.
If the achievement does not unlock when you first control A2, you most likely have experienced a glitch. In this event, clear the cache. To do so, turn off the console by holding the
button on the console for at least ten seconds and then unplug the console for at least 20 seconds. Load a save you made before controlling A2 for the first time. Clearing the cache will most likely remedy the problem and you should earn the achievement following the cut scene when you assume control of A2 for the first time.
If the achievement glitches and you took precautions by making the save during the Loss main story mission as I recommended above, the remedy is very simple regardless of your previous game play. Load up that save after you have cleared the cache. Cross the rope bridge and let the cut scene play out. When you are asked who you want to support, choose A2. Since you have cleared the cache, the problem should be remedied and the achievement should unlock. If you have a save file made later than the Loss save file, you can load it and resume your game play.