Her

Honestly, she had no clue why she was doing this. Most probably, it was to make someone learn from her mistakes or simply laugh at her life. Or she was just crazy. When she started, it was purely because maintaining a diary was boring and there was a chance that the people around her would know what she thought about them. This was the simplest way to maintain a diary without maintaining it. Basically, it was no strings attached.

One day, he does something crazier and replies to this weird person who liked leaving her life stories around in an open book.

Soon, this acquaintanceship (if you can call it that) blossoms and strings attach themselves quicker than you can say 'her'.