According to a bit of dialogue in Animal Crossing: Wild World, Sable has a history with Tom Nook. If the player manages to befriend Sable before January, her mood will shift during the month. Her sister, Mabel, will comment about Sable being in a funk, prompting the player to see what’s going on.
Upon talking to Sable, she will reveal that she and Tom Nook were best friends before he moved to the big city. Sable will reminisce about their days climbing the roof of the observatory to stargaze and how she would write to him to keep his spirits up. One special memory Sable brings up involves Tom buying her a pair of expensive scissors and sending them to her as a birthday gift.
Although it’s implied that Sable may have romantic feelings for Tom Nook, this relationship is never expanded on in later Animal Crossing entries. But in a small bit of dialogue in Wild World, Tom admits that he “lost more than just money, he lost a friend.” This friend is more than likely Sable, since the two never really interact with each other during the game. Hopefully this gets fleshed out more in a future Animal Crossing.