Dave Lester was a young man who nearly crashed his motorcycle into the Doctor's TARDIS while riding one night. After introductions were made, Dave asked that the Third Doctor take him to Carradine Estate, having heard rumours of strange things going on at the place. Once there, they were captured by Professor Rayner, but escaped and together defeated his Master Plant. (COMIC: The Plant Master)
Though Lester wouldn't ordinarily rise to the level of a companion in most periods of Doctor Who, he qualifies at least as an assistant in the Third Doctor's more Earthbound era. He is one in a series of adolescent boys who accompanied the Third Doctor for a limited time.