Disclaimer: Harry Potter belongs to Ms. Rowling. Anything you recognise is probably a coinicidance, but if you have any questions feel free to ask.
So that's how he met his daughter. He was shocked, but she explained everything to him and it all made sense.
:FLASHBACK:
"I Mr. Black" she said triumphantly, standing up to shake his hand, "Am Nicole Iris Black."
"What?" Sirius managed to say. Nikki smiled at him.
"I am your daughter."
"No" Sirius said "No, we didn't have a child."
"yes you did" Nikki smiled at him. "She found out she was pregnant after you went to Azkaban"
Sirius sat in thought. He had so many questions, but the most important thing was he just found out he had a daughter. It would have been nice if Dumbledore warned him, he didn't really know what to do. Sirius pondered his next words. "Who...was that man at the house this morning?"
Nikki's smile faded. "He's my step dad."
"Is he the one who gave you those bruises?" Sirius muttered.
She stared at him wondering if she could trust him. She shook her head yes.
Sirius looked down. "When did it start?"
Nikki sighed, it would be good for her if she shared some of her secrets but not all. "Mom married him when I was four, everything seemed perfect except a few fights. When mom died, I was nine, he started to drink and then take his anger out on me. He is a muggle and he never liked the fact that mom was a wizard, so naturally he thinks I'm a freak."
"I'm sorry" he said sympathetically
Nikki smiled at him "It's okay, I planned to runaway anyhow."
"Seems like you've given this much thought" Sirius said with a grin.
"Naturally" Nikki said sharing his same grin.
"Well there will be no need for that. I can't let you go back there."
Nikki froze. Did he mean what she thought he meant. "You mean I can live with you?"
"Naturally" Sirius said grinning. Nikki smiled joyously back at him.
"Well that's good, now I have someplace to put my stuff" she said lifting her bag up so he can see. Nikki had gotten into a fight with her step dad before she left for the diner. She was so tired of his crap, that she packed her favorite things and planned to runaway tonight after she met Sirius.
:END OF FLASHBACK:
Now things weren't always great between Sirius and Nikki. They fought a lot and Sirius didn't know to respond to these fights, he figured they were what teenage girls did. He didn't realize that he gave Harry more attention than Nikki, it was just that Harry was always in some sort of trouble and Nikki always did everything herself, he never got mad at her. But Sirius still remembered the day that everything changed for the better.
:FLASHBACK:
"So Harry I was thinking me, you and Ron could go to Ireland this weekend and hang out, do guy things, you know? what do you think?"
"Ah, that would be great Sirius" Harry said with a smile and a plate full of food in his mouth.
Nikki stared in awe, Sirius hadn't asked to do anything with her since she got back let alone talk to her. "hey Sirius, can I come too?" She asked in a low voice. Sirius looked over at her as though she just appeared in the room.
"Actually, I was hoping for it to be a guys thing-"
"Right" Nikki cut off. "I get it-"
"Well there's nothing to get, Nikki, we'll do something when I get back, I just figured that me and Harry would need some time alone just to talk and-"
"You don't have to explain! I Get It!" Nikki said raising her voice. Sirius, in an attempt to calm her anger said "Nikki" sternly, hoping she would get the hint.
"What!" She yelled standing up. "I don't care if you don't want to get into this now, because we're going to! Why would you invite me to come live with you if you had no intention to take care of me-"
"I take care of you!" Sirius said now standing up too. "I feed you, clothe you, put a roof over your head-"
"And I had all of that before"
"Yeah, except before you lived with a man that beat you" Sirius said. Nikki glared at Sirius.
"Well, I rather be back in that house with that animal than living with my supposed to be father who acts like I don't exist." This wasn't true, Nikki would never want to live with her step dad again but all the same, it hurt to feel like she didn't matter to her real dad. After she said this Sirius looked a bit shocked, he couldn't believe that she felt that way.
Sirius wasn't sure what to do, but knew that he couldn't let her think she won this fight. "Go to you're room we'll talk about this later" he said quieter.
"No" she protested, "You can't tell me what to do-"
Sirius began to yell before he was cut off. "I am your father-"
"My father who has been in Azkaban for the last 13 years-" Nikki screamed back.
Sirius lifted his hand to hit her, to stop the words that were coming out of her mouth, to swipe away the anger he was feeling. Nikki flinched and Sirius caught himself and his hand stilled in the air.
"Do it, I dare you.." Nikki pressed on. "You'd be just like the others" Nikki could see the anger in Sirius' eyes. Nikki and Sirius appeared to be having a stare down until Sirius began to lower his hand. "I hate you" she said, pronouncing every word slowly. She pushed him out of the way and ran up stairs.
Sirius looked at Remus and Remus looked at him back. He had a look of sympathy and confusion in his eyes. Sirius knew that everything Nikki said was not what made her go off, it was much more than that.
Nikki ran into her room and shut the door, she ran into her art room and went out on the balcony and cried the hardest she'd cried in a long time. She loved Sirius so much but things felt too complicated. She wanted him to love her and want to be with her constantly to make up for lost times. She wanted to be able to tell him her feelings or what it's been like for her in the last few years. She wanted to tell him so much but things just seamed to different.
Sirius entered her room and she wasn't in sight. He heard a low moan and looked in the direction.
"Tut, tut" Phinigus said "What could you have done to upset her so much, haven't seen her this upset since she arrived."
"Shut it Phinigus" Sirius said. "Were is she?" Phinigus pointed to her art room, the door was cracked open. Sirius opened the door slowly. The sight he saw before him was amazing. The room was beautiful. There wasn't anything extremely special about the room except that she had paintings every where. They didn't seam like the paintings a 16 year old girl would paint. They looked professional with so much life and meaning and feeling in all of them. Then his sight came over Nikki crying on her balcony hugging her knees. Sirius approached her slowly.
"I'm sorry" he said quietly and suddenly feeling weak from seeing his daughter crying before him. He startled her and made her jump at his words. She looked up at him, he had so much love in his eyes.
"When mom died-" Nikki began, more of telling her feeling then directing them towards Sirius. "and my step dad started drinking, it all became a blur. One things for sure, I really didn't feel loved anymore" Sirius sat down across from her, listening carefully. "He always told me that I was worthless and a disappointment. I was never good enough and that everyone would always think I'm a freak. Sometimes when he was really drunk, he'd come into my room and try to grope me or lay with me but I'd always run from him ,I knew he'd forget in the morning, but I was always scared that maybe I wouldn't be able to fight him off next time. I always wished that someone would come and save me. Mom had always told me that you were innocent so when I found out you escaped Azkaban, I prayed every night that you would come and find me, but then I'd always tell myself that it wouldn't happen because you didn't know that I existed." Nikki paused and looked at the sky it was so beautiful.
"But then I came" Sirius said quietly.
Nikki smiled to herself. "Yeah, you did. I knew I recognized you that day." She looked at Sirius and smiled, and he smiled back at her. "I thought that we would have this awesome father-daughter relationship and we'd love each other so much-" tears began to form in her eyes, she just wanted to cry without much of a reason.
"It just takes time" Sirius said, hoping she wouldn't cry. "I'm new at this and sometimes I just need someone to tell me what to do. You seamed so independent always doing what you wanted so I didn't think you wanted me to fret over you, Harry on the other hand is always getting into some sort of trouble."
Nikki was feeling a little over emotional and began to cry a little harder. "I know, but can you fret over me once in a while, just get on my ass and tell me I need to do my homework or clean my room" she said with a chuckle, Sirius laughed too and pulled her into a tight hug. Nikki let out a muffled sigh.
"I love you...dad" she said quietly. Sirius froze. Besides the fact that she 'I love you' she called him dad. After a silence Sirius responded, "I love you too"
:END OF FLASHBACK: