I always imagined Charlie as this unapologetic good hearted animal loving human with an intense fascination for nature. Although he, like all Weasleys, is very talkative and approachable, he is so involved with caring for animals and magical creatures that he just naturally surrounds himself with orphaned creatures, no matter their fangs and claws.
Aged 8, he was the first one to move out of the burrow, because Percy was allergic to the cats and he had brought home a litter of street cats with a pinch of Kneazle. So he, together with his cats, moved out into a room attached to the barn and he never moved back in.
He cried when he realised that he could not bring all his animals to Hogwarts and even thought about running away. But when he got there he realised that Hagrid was more than happy to host some of his animals for him to visit. What Charlie had not thought off was that Hagrid got so used to them that it almost broke his heart to take them back to the burrow over the summer. Especially one pup’ got so used to his new owner, never leaving his sight, that Charlie could not bring himself to separate them, and considering the dog’s predisposition for drooling - it was probably best.