If you think something might be socio-emotionally amiss with this person, it can be fun to flip through the Diagnostic and Statistical Manual of Mental Disorders and just slap a diagnosis on them like Borderline Personality Disorder. Some adjectives related to common BPD behavior include:
- Passive Aggressive
You might also describe this person as being in denial. Someone who is in denial of responsibility will often resort to minimizing or blaming their behavior.
"Minimizing" is an attempt to make their actions appear to have been
harmless. They are minimizing their behavior when, after doing
something awful, they say, "It was no big deal."
"Blaming" is a statement that shifts culpability of a consequence. A
person could be said to be blaming you if, after doing something
inappropriate, they say, "What are you so mad about? You must have
gotten up on the wrong side of the bed this morning! Don't take
everything so seriously!"
It's also possible that you are dealing with a narcissist, either in the pathological schizotypal sense, or just the 'typical American teenager' sense.
Colloquial, non-psychodiagnostic terms that could apply here include saccharine, fake, conniving, disingenuous, nonchalant, unphased, glib and shallow.
Right off the top, however, the first adjective that sprang to mind for me, was bitchy.