So this is for Destiny Xavier16, hope you like it and thank you for the idea :D
Kate smiled to herself as she looked out of the window, the warm sunlight shining over her. She had been living with Castiel for just over a year now, since he had fallen from heaven and pretty much become human, she had completely fallen for him. He was almost like a completely different person when he was with her, just as she was when she was around him. Her small smile quickly turned into a grin as two arms snaked around her waist and pulled her back slightly. "Hey you." She said lightly, leaning her head back enough so that she was looking up at Castiel's face.
"You look beautiful today." He said softly, turning her around in his arms and kissing her gently.
Kate laughed slightly before she kissed him back, pulling away after a minute and raising an eyebrow to him. "What's gotten into you?" She asked him curiously, sounding amused.
"Nothing." He stated simply, gently playing with the ends of her hair at the middle of her back. "My girlfriend's just beautiful."
"Cas-" She began, Castiel cutting her off quickly.
"-and amazing, and sweet, and adorable, and captivating and.." He trailed off and tilted his head to the side, a small smile on his face. "Kate, are you blushing?"
Kate laughed and pushed him gently in the chest. "Stop it." She pressed lightly.
A slight smirk formed on his face as she looked away from him. "You're blushing."
"No I'm not." She argued, keeping her gaze fixed on the floor.
"Yes, you are." He smiled down at her, his eyes fixed on her face as she laughed to herself. He stopped laughing himself, his face turning serious as she looked down at the girl he'd fallen in love with.
Kate let her smile fade, seeing the look on his face. "Cas? You okay?" She asked him, her voice sounding a mixture of confusion and concern. "You look terrified."
Castiel shook his head and took a hold of one of her hands. "I need to ask you something."
"Okay." She said slowly, obviously not having a clue where he was going with this.
"You know I love you.." He began, taking a deep breath as he trailed off.
She let out a small laugh and frowned in confusion. "Yeah, of course I do. Cas, what's going on?" She asked him again, getting slightly worried at the nervous frown on his face.
"Good." He said simply, squeezing her hand slightly before bending down in front of her.
"Cas, what are you-" She began, stopping when she realised what he'd pulled out of his pocket, her green eyes going wide.
Castiel opened the small box, showing her a ring. "Kate Winchester, will you marry me?" She stared down at him for a couple of minutes, her mouth hanging open until she blinked hard, his question finally sinking in. A smile spread across her face as she nodded, using the hand her was holding to gently pulling him up to face her. Castiel grinned at her as he rose to his feet, leaning in and kissing her passionately. He pulled back and took the ring from the box, smiling and sliding it onto her finger. His gaze moved from her hand back to her face, reaching up and gently resting his hands on her cheeks, wiping the tears that were falling silently down her face away with his thumbs. Kate laughed and wrapped her arms around his neck. "You know, now you've said yes.. there's the hard part to deal with." He said to her slowly, resting his forehead against hers.
Kate looked up at him and raised an eyebrow. "Oh yeah? And what's the hard part?"
"We've got to break it to your brothers that their baby sister's getting married."
She thought it over for a minute before a smirk spread across her face and she laughed. "Well, good luck with that." She said lightly.
"Maybe we should just call them?" He suggested, glancing over at the phone.
Kate laughed again and placed a hand on his cheek, turning his face to look at her before she kissed him softly. "Sam and Dean are on a hunt, and besides, there's plenty of time for us to tell them. Right now, there's only one person that I wanna be with."
Castiel moved backwards to lie on the couch, pulling Kate towards him and holding her in his arms. He stared up into her eyes and smiled, pushing a lock of blonde hair behind her ear. "You're so amazing." He said quietly. "I'm going to love you forever, I promise you."
"I'm going to hold you to that." She smiled as she ran a hand gently through his hair and rested her head against his shoulder. "You know something, you're absolutely everything to me Cas.. I love you so much." She whispered, placing her hand flat on his chest beside her face and looking at the ring on her finger.
Castiel kissed the top of her head and let out a content sigh. "I love you too Kate." He said softly, tightening his arms around her.
Thanks for reading guys!