Losing her father, so to speak, to the horrible woman she hates, came as a terrible blow to her. Although she was never the most extroverted of chidren, that was the turning point that brought Darby in upon herself. She's not shy in the least, but she's also not the most open of students.
Darby is a hardcore cynic. She's seen the world offer her nothing but bad breaks and the like, so she expects little else. Her prose and poetry, therefore, tends to run to the dark, gothic, and rather depressed side of things. In speech, she uses her eloquent, stuffy manner of speech to further keep folks at bay. For those few who bother to try and see past this aloof exterior, they would still likely find her unyielding to any attempts to probe her thoughts.
Above all, Darby longs for company. Although she claims to prefer her own company, and fights against letting anyone into her private world, she is actually aware of how drab and depressing it is. She envies those who can make quick and true friends right off the bat. But she just cannot allow herself to take the risk of letting someone in just to hurt her again.