AU. Set after S3 but Knockout never happened.
A spark died in him the day he saw the ring on her finger. For a long time, far too long, she didn't notice. She had feared that, as he had once before, he would leave when she was no longer his alone and the fact that he remained with her was enough. Enough to convince her that she had made the right choice; enough for her to pretend that she hadn't changed everything between them.
He was still there: still by her side, still bringing her coffee and still spouting crazy theories. Still covering her back and following her wherever she went and still building theory as though they were two parts of a single whole. Still listening whenever she needed to talk and, though she didn't realise it, still keeping the promise of 'Always' in the only way she had left to him. He even still teased her, still pulled her pigtails a little and if the flirting stopped and he no longer asked her about her plans when she was off duty, well that was hardly significant, he was just respecting her relationship.
The others saw it and pulled closer around him, trying to shield him from her unintentional cruelty. Whereas they had once closed ranks against him on her behalf, they now fought to hide their anger with her for her blindness. They wanted to speak, to tell her what she was doing but he wouldn't allow it, wouldn't let them do anything that might jeopardize her chance at happiness. Despite his warnings, they couldn't help the slight distance that formed, though they tried for both their sakes, and the banter in the bullpen became stilted at unexpected moments when it brushed too close to the elephant that took up most of the room.
For a long time, she didn't understand. Slowly, almost imperceptibly, she began to realize that something wasn't right but she couldn't quite see it. Couldn't see that she had almost killed the joy and childlike optimism he had once brought to everything, replacing it with resignation and regret. The belief in magic he had retained throughout his life had all but faded away.
She didn't realise that he had lost his smile until she caught a momentary flash of its old radiance directed at his daughter. The unexpected brightness of it came as a shock and her stomach twisted as she tried to recall the last time he had truly smiled at her and realised that she couldn't. The bright, beaming, often mischievous smile that had lit his face from within and always made her want to smile back was gone. For months it had been replaced with smaller, more restrained smiles that never quite took the shadow from his eyes.
It was as if that first realisation opened the floodgates and suddenly the myriad of changes were overwhelming. Every time she looked at him, there was something new and painful to see. From the fact that he seldom met her eyes fully any more and almost never for longer than a few seconds to the understanding that he was spending more time with the others and less sitting by her desk watching her do paperwork. When a case was closed, he left earlier and, though he still occasionally suggested a drink to celebrate, his invitations were always for the whole team and never just for her. Most telling of all, though, was the sadness in his expression when he thought no one was watching and the speed with which it was hidden when he knew he was being observed.
She saw it and she hated it but she still didn't understand. She had closed her mind to the possibility of them being more than partners so firmly that the grief she was causing did not compute. She tried to talk to him about it once but he brushed her off, assuring her that she was reading too much into it and that the sorrow she saw was simply him missing his daughter. After that, he acted more like his old self - clearly having inherited more from his mother than she had realized - and she would have bought it but for the brief moments when the the cracks in his mask showed. The moments when he betrayed himself were almost imperceptible but, now that she was looking for them, just enough for her to recognize that they were there and the fact that he felt he had to hide from her caused a sharp stab of pain in her chest.
The others looked at her in disbelief when she asked if they had noticed anything wrong with him and refused to break his confidence, although in truth he had confided in them very little and they hadn't asked, knowing the cause of his pain and understanding that words couldn't help him.
It was a bright flash from her ring, as her gesturing hands made it catch the light, that finally betrayed him. Caught of guard, he couldn't conceal the anguish that filled his eyes and, for a split second, she could see everything that he was feeling. Then his gaze shuttered and he picked up the thread of her theory and continued to spin it to its logical conclusion as if his pain had never existed. Her hands dropped slowly and she had to struggle to concentrate on the case as she unconsciously began to twist the shining ring on her finger.
How long, she wondered. How long had her partner been bleeding out in front of her without her ever even noticing that he was wounded. Worse, without seeing that he was dying by inches from a wound that she had inflicted, keeping silent about his pain so he could to give her whatever she needed from him.
For the next few days, she was uncharacteristically subdued, resisting all efforts for draw her out or lift her spirits, her hands playing restlessly with her ring every time her thoughts drifted from the case they were working on.
"I wish that I could have someone who would be there for me and I could be there for him and we could just dive into it together...". Her own words echoed in her head, repeated over and over again, pounding at her until the truth was undeniable. He was there - through everything -giving her strength when her own was not enough, giving her joy when the darkness threatened to drown her, supporting her, listening to her...loving her without reserve or thought for the consequences to himself. He was there...but she never had been. She took everything he offered and gave nothing but crumbs in return. Doling out carefully measured pieces of herself, always pulling back before she revealed to much, slapping him down when he got too close and it was all so futile. The missing details of her life did nothing to reduce the clarity of his vision. He knew her. Better than anyone ever had – he saw right down into the depths of her and he loved her anyway.
On her day off, she shut herself away with her thoughts and finally allowed herself to really think about everything that had happened since he came into her life. How much had changed and how much he had changed her. How much better he made things just by being there. And when she returned to the precinct the next day, her head was high, her mind was finally clear...and her ring finger was bare.
After everything that happened this season, I can see Castle staying, even knowing that Kate is engaged, because he'd rather endure the pain of seeing her happy with someone else than risk something happening to her because he wasn't there to watch her back.