Miku always told the truth. She just didn't say some out loud.
(Does that make her a liar?)
She didn't think it would change much. Everything else was a lie.
(Does that make her a bad person?)
Miku always apologized when someone got angry at her for not telling them everything.
(She wasn't sorry for what she did; she was sorry that they believed her fully in the first place.)
Everyone was a liar anyway, in some way. It wouldn't matter to add a few more, right?
(Maybe it would, but at this point, Miku didn't really care.)
Okay, I realize this chapter (and probably the last two) were somewhat confusing. Parentheses, in this case, represent the thoughts they don't want to say.
Miku is getting a little development in this chapter; Teto is not getting too much development quite yet.
In Miku's last chapter, she was a bit more paranoid. She asked more questions and she worried a bit more. She brought some of that into this chapter, but she's already changing bit by bit. "...at this point, Miku didn't really care," is the hint that she's becoming more accepting of the dark world idea. Teto was already there.
...I probably should have done a better job of portraying this, and I'm sorry. I'll try to improve in later chapters.