Tom Phipps

 was an English schoolboy who once assisted the Third Doctor during the latter's exile on Earth.

Tom had inherited a love of birdwatching from his naturalist father, Professor Brian Phipps, so he was able to guide the Doctor on a trip in Bessie out into the countryside around Tom's house. During the trip, the duo encountered what appeared to be a freak tornado. On returning home, they discovered that the Phipps' house had been completely destroyed by the tornado.

The Doctor realised that the "tornado" was too interested in him, specifically, so he decided to distance himself from the Phipps' family and spend the night camping on a remote hill. Unbeknownst to his parents, Tom went to the Doctor's campsite just in time to be sucked up into the returning "tornado". The vortex turned out to be a conduit to an alien space ship in orbit above the Earth. The two rapidly worked out that they had been captured so as to form a part of a menagerie of different life-forms, already onboard. Not wishing to remain held up in a zoo, they worked together to understand the unusual biotechnology of the vessel. Together, they figured out how to "reverse the programming" and get themselves re-deposited on Earth. (TVA: The Vortex)

Behind the scenes
By the standards of most eras of Doctor Who, Tom wouldn't qualify as a companion. But for the exile on Earth period, he's quite comparable to Liz Shaw or the early season 8 Jo Grant. He begins the story helping the Doctor on a question of scientific curiosity, then suddenly is whisked into an adventure with Earth invaders.