9S is the second playable character. You have to start the game with the same save-file you completed the first part (2B).
You can use self-destruct to blow your pants off, but there is no need to use self-destruct:
You can buy the clothing module from Emil. Emil becomes available later in the story
(
).
It costs 10.000G and is a permanent item.
Option 1: You can equip it, so your clothes stay in good condition.
Option 2: You can use it. Using it removes the lower parts of your clothes. This is the same effect that you get when you use self destruct. But there are no negative effects (one-hit death, grey screen)
Clothes modified by this stay the same between your playthroughs. If you finish your game with 2B with this mod active, she keeps running around in her pants the whole time while playing with 9S. Keep that in mind, if you like OR do not like this!
TL;DR:
Option 1: Self-destruct
Option 2: Clothing module from Emil
Option 3: Buy from "achievement-trader" after you've finished ending C/D