Collating and combining answers from various asks, with some bonuses in there too. Please feel free to contribute ideas, if I like them I’ll add them*~

• Castiel is demisexual. This is like a parking spot between sexual and asexual; he doesn’t care for gender or appearance, but solely develops sexual attraction based on emotional connection. Once a deep relationship or friendship develops, he feels sexual attraction - and not a minute before. 
• On the Kinsey scale (which is outdated at this point - it only applies to two genders, and this player knows very personally there’s more than the binary) he rests in the middle perfectly, with no preference to men over women or vice versa.
• Castiel is not in fact male; he’s agender, but in his opinion the vessel he’s in defines his gender. If he were to possess a woman, he would be a woman. As it stands, he’s possessing a man.
• Jimmy’s spirit moved on a long time ago. Castiel has his memories, but Jimmy himself is gone, moved on to paradise. The body is a reconstruction at this point, having been put back so many times by God. Castiel is more closely tied into it than he normally would be a vessel, and feels everything as if it’s his own form. If Castiel were to leave the body now, Jimmy would be brain dead.
• Castiel’s favourite food is not in fact burgers, but fresh fish. As an angel he doesn’t need to eat bar in exceptional circumstances. Burgers are a craving given to him by his vessel, not a personal taste. The taste of fresh, steamed fish is preferred above anything else. It’s one of the cleanest flavours.
• He has a multitude of fears. It was the first emotion he really felt (with doubt following very quickly), and since he took on Sam’s pain and recovered, he still feels the affects of being a little more fearful than he should be. His greatest fears are being useless, Dean dying permanently, Alastair (in general), and being cast out.
• Very occasionally, he still sees Lucifer, in flashes as opposed to full blown hallucinations that speak to him. He has never fully recovered from taking on Sam’s pain - it’s still there, just locked away.
• He doesn’t react to temperature change, unless he is weakened. However, his body temperature itself runs hotter than a normal human.
• If and when he sleeps, he is prone to nightmares about drowning. Dean is often a prosecuting figure in his nightmares.
• He is approximately 100 million years old as he remembers pre-human life (the ‘missing link’, the fish that pulled itself onto shore), but he’s very young as angels go - surprisingly young for a garrison captain, which proves just what a diligent soldier he used to be.
• Castiel loves hard liquor, but he has a sweet tooth for lighter drinks.
• Castiel’s favourite animals are wolves and bees. He likes the pack and hive mentality of the species, and bees are just fascinating generally.
• Although Castiel fell and gave up Heaven, he doesn’t want to be human. It would take an awful lot for him to want that.
• He does have wings. They’re a brown a shade lighter than his hair. If you break his focus in the right way, they will materialise. Think of it like breathing - they’re out of sight unconsciously, he does it without thinking about it or having to.
• He doesn’t think very highly of many Biblical figures - save Ishmael (who Castiel knew personally), who is the ancestor of the Novaks.
• It’s on his ‘bucket list’ to go fishing with Dean.
• He wants to be trusted and forgiven above anything else. Forgiveness is powerful for him. He has a long list of his wrongs and regrets.
• No, he does not understand that reference.
• Dean and Crowley are his weak points.
• He knows all of his angel siblings, even if only by name. Not all does he know personally.