When Merlin was in the army, he was often surrounded by dogs. Whether they be bomb dogs, or simply ones there to offer comfort when they got back to their base. He was never overly fond of them, as he had grown up with cats and the fact that they needed seemingly constant attention. But at the time, they were there and he would admit (albeit after a few drinks) that one of them had a special place in his heart (it was after a particularly bad raid, and the puppy wasn't quite trained to be a solid source of comfort, but that's a story for another day).
When he was discharged, and left mostly to his own devices, he contemplated getting a dog as a companion. It would give him something to focus on when he wasn't busy, he reasoned as he went into countless shelters trying to find the right dog. In the end though, he found it brought back memories that he didn't quite want to remember, and couldn't bring himself to actually adopt one.
His answer to his loneliness however, came in the unexpected form of a brown and white ragdoll cat he found while visiting one of his friends. Harry, as Merlin later dubbed him, was in a small adoption place that was out of the way quite a bit. At the first sight of Harry, Merlin knew that he found his 'companion' that Percival was always on him about getting. Within the week, Merlin was back home and getting Harry settled in as well as he could.
It wasn't very hard to adjust to life with Harry around. Merlin would go off to work at Kingsmen (thanks to a few strings being pulled, he got the job almost immediately after being discharged), work himself entirely too hard and for to long, and come home in the evenings to relax with Harry. Percival liked to joke that Merlin liked his cat more than anyone else, human or otherwise, and Merlin couldn't really find it in himself to argue. Harry was there for him no matter what, though Merlin couldn't say that he didn't get judged by Harry, because he certainly did. Constantly.
Harry was also good at telling which people were good enough to be in Merlin's life, and which ones definitely didn't. He often showed the latter by either ignoring the person completely and being all over Merlin, or growling at the offender. There of course was the incident regarding Arthur, where Harry went as far as to bite the man's hand and leave scratches all down his arm. Other than that though, Harry was very laid back with people he deemed worthy of it and Merlin learned to trust his judgement after less than a month of having him.
Merlin eventually got into the habit of letting Harry venture outside every once in a while, though not until he made sure the cat had his collar on, and was fed. It was usually right before he went to work that Merlin would leave the window open slightly, if the weather permitted it (it was times like these that he was thankful he had gotten a house instead of just renting a flat stories above the ground). He'd tell Harry to be sure to come right back home after he was done exploring, and depart with a kiss on his head, that Harry always welcomed.
He was always slightly worried about Harry on those days, but he was always back and laying on Merlin's favorite chair by the time he got back home. There was only once when he had gotten a call from a concerned person who had found Harry wandering in their backyard and wanted to make sure he knew his cat was outside. Aside from that, no one really bothered Harry or decided to call when they found him, as he must have seemed to know what he was doing.
Though, with doing this, and letting Harry outside, Merlin really should have expected the day when he came home to find not only Harry in his living room, but another cat, well more of a kitten. A very dirty, scrappy kitten. The kitten had hissed at him when he entered the room, but Harry hit him lightly with his paw as if he was telling the other cat to shut up. Merlin approached the cat slowly, and took note of how scraped up it seemed to be; a piece of its ear missing at the very top, along with a long scar that ran down its side. Apart from those though, the cat was gorgeous, it was a short haired tawny, calico. A pretty common breed, but Merlin saw the uniqueness in him.
The kitten eventually let Merlin close enough to make sure he wasn't hurt at the present, though he was still wary of him. Merlin looked up at Harry and told him that he was going to be cleaning the kitten up until he trusted Merlin enough to.
After a few days, Merlin started calling the kitten Eggsy, as he had somehow gotten onto the counter and managed to bat the three eggs Merlin had waiting to be put into the dish he was making, onto the floor.
It was about a week later, when Eggsy finally started to warm up to Merlin. He let Merlin scratch at his head while looking him over more thoroughly. Merlin wasn't exactly a vet, but he had the basic skills needed to make sure that Eggsy wasn't physically hurt. Their relationship slowly continued from there, with the added help from Harry of course, who refused to be ignored by either of them. Merlin found that they were eerily similar in that respect.
Eventually it got to the point where Eggsy would hop onto his chair with him and bat at his arm until Merlin lifted it up enough for him to squirm underneath it, and come out against his chest, half on top of Harry. It was like Eggsy had been there from the beginning with Harry, he fit in so seamlessly to their dynamic. Eggsy even despised Arthur, as Harry did (Harry, and eventually Eggsy too had to be put in Merlin's room whenever the man came over, because as much as Merlin loved them, and hated Arthur, he didn't want the man to complain about them to animal services or something of the like).
One day, while sitting in his chair, reading a book with Harry and Eggsy sharing his lap, Merlin realized just how much his life was changed by the two of them. They had shaped his world from the army and the repercussions he suffered due to it, and the loneliness that came with working and coming home to an empty house, to feeling somewhat whole again, and a with a more concrete sense of belonging. Harry and Eggsy were his darlings, and he couldn't imagine life without them by his side, keeping him happy and content.