Well I don't really have the time for a long campaign post I fear, but I will say a few things at least. Like mentioned earlier, it's a bit difficult to talk about 2B, because you enter massive spoiler territory if you aren't careful.
What I can say is that 2B appears stoic on the outside at first, think Mikasa Ackerman (they even have the same Japanese VA
). Just by a quick look they share a number of traits. Stoic, strong-willed, a very capable fighter. She's more independent though, in the sense that she isn't fixated on one guy, like Mikasa is. But I don't really like comparing characters, so I will talk about 2B specifically.
What's the most important thing to her is protecting humanity. She's a part of YorHa, an android unit made to protect the human race. Their credo is "For the Glory of Mankind", and that's what they live by.
Their opponents are mainly machines, as said before. As you probably can imagine, a huge theme of the game is what exactly makes a person human or a machine, but it also questions if being human is actually a good thing/desirable. YorHa units aren't allowed to show emotions (one of the reasons they wear a blindfold), and they put great emphasis on the fact that they are not humans but inferior soldiers meant to protect them.
This is something 2B struggles with a lot, during the whole game. On the one side, the mission to protect mankind, the very reason they were created for, and on the other side emotions that she desperately tries to ignore because they could get in the way of their mission. A lot of this isn't spelled out in the game directly, but always hinted at, even in the beginning of the game.
That's why slowly but surely, she becomes a very complex and fascinating character. If you look below the surface, she's playful and kind, but the state of the world doesn't really allow her the small happiness she wants. I realize I'm being pretty vague here and maybe I will go into a bit more detail later on, but it realy is very very difficult to say much without big spoilers
(yes, I know I'm repeating myself)
The game actually has a song dedicated to her character, so that might give you some further insight while only hinting at certain elements. And it's a great song, too
Additionally, here's some nice fanservice for those who prefer the looks
And if you're a bit more curious about the game, here's one of the best tracks (the OST is amazing):