Her father

New client

The ex-soldier sprang to attention as a familiar face came through the door to his and Sherlock's apartment.
There stood James Moriarty, grey Westwood suit, as usual.  Sherlock hadn't looked up from his newspaper having figured out that Moriarty was coming up the stairs a while ago.

When he finally did, he and Moriarty shared a look.  Slowly, delicately, Sherlock folded his paper and placed it on the table in front of him.  He indicated for Moriarty to sit in the kitchen chair that sat between he and John's armchairs.  Moriarty sat it what had come to be known as the clients chairs willingly.  Knowing that that was what he was.  A client of Sherlock Holmes.

John continued to be confused at not only how Moriarty and Sherlock had communicated but also at the sheer fact that Moriarty was a client.  Sherlock just indicated for John to sit down and eventually, unwillingly, John agreed.

"My daughter has gone missing" Moriarty stated calmly, despite the panic that was still building inside of him. This surprised both John and Sherlock as neither of them knew he had a child. Sherlock held up his hand to stop John from spilling his presumably countless questions. Sherlock then nodded to Moriarty, indicating for him to continue.
Moriarty pulled a picture from his inside jacket pocket of a young girl, his daughter. He handed it to Sherlock who had to continually look away and look back to be able to catch his own thoughts.
The girl was fifteen, long wavy brown hair, pale skin, freckles. Her name was Arrabella. None of this Sherlock got from the photograph but because he knew the young girl that he had been sent to find.

Sherlock placed the photo down on the table in the middle of the three. John looked at it and his eyes widened in shock and realisation.
"Problem?" Moriarty asked, a touch of anger in his voice. John coughed to compose himself then shook his head saying no.

"Good. Her name is Arrabella or Bella for short. She is fifteen. Last I heard from her she was going out to meet her girlfriend Ellie, who I also haven't been able to track down." Moriarty explained. His eyes met Sherlock and in them shone just a glimpse of emotion.
Heartbreak. Moriarty was truly and deeply saddened by the loss of his only child. It was this emotion that pleaded with Sherlock to take the case. Silently, Sherlock nodded his head, agreeing to take the case.
Moriarty understood and he left.

They waited for the door to close downstairs before John let out a long breath then turned to begin asking Sherlock questions. Sherlock himself was trying hard to keep together when inside he was panicking to no end.

"That's Bella." John stated.
"Yep." Sherlock replied, popping the p at the end.
"No but Sherlock. That's Bella, our Bell!" John was standing at this point.
"Yes John I know! I know full well that that is Arrabella!" Sherlock screamed back. The two took deep breaths. John sank back into his chair and Sherlock went over to the window as a tear slid from his eye.

"Arrabella Moriarty." He said under his breath. "My child is dating Moriarty's daughter"