Dean and Ledan (known as halechester) have been in a somewhat randomly formed romantic relationship for just over 4 years.
They started off, technically, as enemies and made their way through a bizarre and untrusting friendship into a romantic relationship. They met just over 11 years ago, when Dean and Sam mistook Ledan for a dark witch and attempted to kill him. They tried to do this and it led to Leda faking his death so that they'd leave him be, under the impression they'd succesfully dealt with their case and killed him.
The next time they met, he ended up explaining that he wasn't what they thought, not that they fully trusted him, or even slightly. It was only part way through the apocalypse that Dean came clean about what he really was. Explaining that he didn't like to advertise the fact he was a specific kind of nephilim, due to nephilim in general being hunted by angels due to their "abomination status". He also offered to help them. Sam and Dean, though kinda suspicious of his motives, agreed to this—— though not trusting Leda one bit, they needed all the help they could get.
Over the course of the apocalypse Leda and Dean got to know each other better, as Dean questioned the nephilim's motives for goung against his "family" and helping humans. He found himself sympathising with the man as he explained how angels hunted him as an "abomination" for what he was, because god didn't create his species. Dean could see why anyone would betray them under those cicumstanes.
Though still not trusting him, Dean came to view Ledan as a sort of friend, calling him on occasion when he needed advice on something witch-related, given that Ledan was somewhat an expert on that particular community. This resulted in them drinking together on occasion when he was in town.
However, it was only when Dean went to Ledan to ask for something he knew was too much to ask that Dean's 'kind of friendship' with Ledan turned into one with any level of trust. When his little sister, Sawyer, was taken over by a powerful entity, he not only asked Ledan to help kill the thing, but to try and do it without killing Sawyer (someone she did not get along with). Needless to say, this would make it a much harder task for Ledan to accomplish. Shockingly, Ledan agreed out of loyalty to Dean, to try.
After this—— Dean felt he reasonably knew he could trust Ledan.
After this, and over the next few months, Dean's visits to Ledan's house became more and more common, and were more and more obviously nothing to do with his experience as a witch or a nephilim; It became clear to them they actually enjoyed each others' company. Eventually these visits became sexual in nature on occasion, which slowly formed into them just admitting they were somehow—— bizarrely—— in some kind of a relationship.
This was obviously to anyone who knew them, let's face it.