An AU where Dean’s love for Sammy is personified as an ancient eldritch being that awoke inside of Dean when Sam was conceived. And, only Dean can see it.
Dean has a love/hate relationship with it. It scares Dean sometimes, a lot when he was a child. But, he begrudgingly comes to respect in later years.
Ultimately, it is Dean, and Dean knows this, but seeing it personified makes it easier to blame and make into a monster when the love is overwhelming and dictates actions and choices.
One thing the being does is never lie. It doesn’t talk, but there are times when Dean will see it try to consume Sam and cage Sammy even though it can’t. And Dean hates that he can’t hide that part of him away and avoid it. He has to see it.
Also, when Sam and Dean fight it often becomes prickly to Dean. Mostly as just a silent staring and judging oppressive presence, only in very dire times is it something that tries to directly oppose Dean. But, as Dean gets older he learns ways to block the creature out. The being is truly a very limited creature yet still can be powerfully moving to Dean.
I don’t know if others have one too or not. Maybe it’s just a blood line thing or special spiritual thing and only limited people have it. Maybe the whole world has one but they can only see their own. Maybe it’s just happens with people that love so big it’s hard to keep contained in one body.