Durarara!! Light Novel Volume 01/Prologue

Synopsis
Seiji Yagiri is aware of being stalked by a girl whom he remembers rescuing from a thug a week prior. Coming home and finding her in his apartment, having seen something she was not supposed to see, he has to get rid of her. Seiji's older sister is happy to come and dispose of the girl's body while promising to protect her little brother.

Unanswered Questions
Who is the 'girlfriend' Seiji refers to? What did the stalker see in Seiji's house? Why does his sister have people so ready to dispose of a body?

Quotes
Seiji: "I'd rather not work with evil people. They turn me evil, too. If I turned bad and got caught by the police, my girlfriend would be so sad. That's the last thing I want."