Relationship with Candace Flynn and Stacy Hirano
He was the love interest for both Candace and Stacy (both are now possibly in relationship with Jeremy Johnson and Coltrane respectively, who both are in Jeremy and the Incidentals). Jeremy and Coltrane possibly do not know who Billy is.
Billy is never seen in the series, and also mentioned only once. What happened to him since is not known.
It is possible that Billy had to leave Danville sometime later the third grade, or he still lives there somewhere, but never talks to Candace or Stacy.
It is also possible that Billy would be living in Danville, but never seen. However, this is not likely to have been happened since Candace and Stacy would have been still flirting with him. It is also possible that he is living in Danville but Candace and Stacy are not flirting because they have found Jeremy and Coltrane.
However, none of these reasons are ever explained.
It is not known of his physical appearance as he never did appeared in the series and it wasn't even mentioned how he looked like.
It is known that Stacy glued a hamster at Candace's hair to impress Billy.
- "Spa Day" (Mentioned only)