Muggles' Guide to Harry Potter/Characters/Cecilia

Overview
Cecilia is only seen once, out riding with Tom Riddle.

Half-Blood Prince
Harry and Dumbledore, examining memories illuminating Voldemort's history, start by viewing memories of one Bob Ogden, as he visits the house of Marvolo Gaunt and his children. As that visit comes to an end, Tom Riddle, mounted on a glossy chestnut horse, and a "very pretty girl" on a grey, ride past the Gaunt shack. The girl, whom Riddle addresses as Cecilia and "darling", is the one to notice the snake nailed to the front door of the house. Merope Gaunt, Marvolo's daughter, is jealous of Cecilia, because she is secretly in love with Tom Riddle.

Relationships with Other Characters
It seems likely that Tom Riddle and Cecilia are at least dating, and possibly are engaged to be married, or nearly.

Merope Gaunt's unrequited (at this point) love for Tom makes her jealous of Cecilia. Cecilia, on the other hand, is effectively unaware of the existence of the Gaunt family.

Analysis
Apart from her name, her apparent beauty, and the fact that she rides a grey horse and is familiar enough with Tom Riddle, the heir to the Manor, to address him as "Tom", and for him in turn to address her as "darling", would indicate that they are on close, if not intimate terms. The fact that they are allowed to ride out together, unchaperoned, in the English countryside in the 1920s, would indicate that they were at least committed to each other, if not formally engaged. Her unfamiliarity with the Gaunt family would argue also that she was not from the immediate area. It is possible that Tom had somehow been involved in the war effort in the First World War, and had met Cecilia elsewhere, and was now bringing her back to see his home.

It is worthy of note, perhaps, that when Tom's body is found some fifteen years after these events, mention is made of his parents also being murdered, but no mention of wife or children is made. We know that Tom was shortly after these events spirited away by Merope Gaunt, and that he returned some time later having been married to her; it is entirely possible that, on Tom's decamping so precipitously, Cecilia had thrown him over and left.