Peter Jones was a Dragon in every series of the British Dragons' Den from its 2005 launch.
Jones was part of the original five-person panel and remained the programme's continuous investor as the cast changed. The first panel also included Duncan Bannatyne, Rachel Elnaugh, Theo Paphitis and Simon Woodroffe. Later series introduced other prominent businesspeople, but Jones retained his place.
His television persona is built around examining whether a product can become a scalable business. In a typical pitch, that means probing sales, profit margins, intellectual property, distribution and the entrepreneur's requested valuation. A successful offer still depends on negotiation after the cameras stop, so an on-screen deal is not simply guaranteed money.
This question is narrowly about continuity within the British television series. It does not mean Jones appeared in every international edition, nor that he was the presenter. He was an investor, and his uninterrupted run is what distinguishes him from the other original Dragons.