A flasher, particularly an old vagrant type.
Local (Roxburghshire) variant of dobber
(a dobbler is a float used in angling, originally a quill with the tip painted red, which bobs up and down)
"See auld Davie the Dobbler's been in front o' the Shirra' again. They shud jist cut itaff."
The "driver" of a bicycle either sits on the seat or stands while pedaling. The "passenger" does not pedal and sits on the seat, rack or handlebars of the same bicycle. This is a doubleride.