From the moment he met her, Castiel knew there was something unique about Jo Harvelle. Sure she was a female hunter, which according to the Winchesters was uncommon, but she reeked of strength. She was intimidating in a way that he knew only Slayers could be.
She probably didn't even know it, but she was a Slayer. Not a Potential, but an actual Activated Slayer.
Very few angels would have been able to recognize her as a Slayer, becuase very few Angels came face to face with an actual Slayer. Before the war began with Lilith and he was still in Heaven he had met a Slayer.
There were rumors about a Slayer who had fought against countless demons and gods but Castiel never thought the rumors were true orif they were that he would actually meet her. But he did. She called herself Buffy, which Castiel believed to be a strange name even for a human, but she was strong and confident. The gods she claimed to have bested were nothing more than lesser Angels who had developed ego complexes, but that was still saying something. Not many Slayers, or any beings for that matter, were able to hold their own against an Angel of any calibur and survive. He was impressed by her. Every day he would visit her in her Heaven and he would listen to her stories of the humans and the battles she would fight. She would talk about her friends, family, the demons, and the vampires. She also told him how she had a normal life until she was called as a Slayer. He found her stories of humanity fascinating. To this day Castiel believed she was the reason he felt so attached to the humans.
But one day she just didn't show up. It's like she had vanished from Heaven. He later found out that she had been restored to her human body on Earth. She was called back as a Slayer.
Somehow he felt Jo could best even Buffy. She was tougher, bolder, and stronger. Fighting demons wasn't forced upon her, it was something she genuinely enjoyed doing. It was in her blood. Or so he was told.
Castiel had learned from Buffy that the only way to become a Slayer was if one had died. It explained why not many Angels had met Slayers. There was only one every generation. That still didn't explain Jo. She wasn't like the sudden rush of Activated Slayers he had been told about. She had been a Slayer for years now. Long enough for her to believe her strength was hers. The only conclusion that he could assume was that Jo had been Activated when Buffy had died for the 2nd time.
But Jo had been fighting demons for so long, she didn't even realize that she got stronger or faster. Jo seemed to be the kind of person that filled the room with joy, and had the ability to make a bad situation good. Dean seemed enamored with her, though Castiel couldn't tell if it was Dean's usual flirtatious self or if he genuinely liked her.
Castiel wished he had more time to find these things out. To see what kind of Slayer she was, and to learn more about her. He had never met a Slayer while she was still alive until now. Though Dean had previously mentioned her and her mother, Castiel regretted not meeting her before now. He was sure she would have become a great Hunter and Slayer.
It's too bad there was a Reaper standing over her shoulder.