Billie Silvey
In his journeys through the galaxy over the years, Dr. Who has picked
up an interesting collection of traveling
companions.  These companions
play the role of Everyman or -woman, giving us as viewers someone to
identify with.

The powers of a time lord make Dr. Who less a character with whom
we can identify.  The fact that he can’t be killed, or if he is, will
regenerate into someone with the same memories and general qualities
but a different face, could easily make him like Superman--fearless,
flawless and unable to be overcome.  This would go a long way toward
destroying dramatic tension.

The companions help make the doctor more human and more protective
of the human race.  Gallifreyan society is cold, rational, and uninvolved,
while the doctor is becoming more emotional.

Requirements for companions seem to be curiosity, adventuresomeness
and the ability to appreciate diversity.  Without them, Dr. Who would
spend a lot of lonely, silent time aboard the TARDIS, which again
would limit drama.  

The first companion who made an impression on me was Leela, the
savage.  Played by Louise Jameson, she brought a naive comfort with
brutality that played well against the doctor's sophisticated morality.
October 2013
Leela (Louise Jameson)
Rose Tyler,
(Billie Piper)
Later, I enjoyed Rose Tyler (Billie Piper)
and her boyfriend Micky Smith (Noel
Clarke), two kids from the London
working class who brought a roughness
yet vulnerability to the companion role.  
Their relationship with each other and
with the doctor added interest and
complications to the episodes in which
they appeared.
Micky Smith,
(Noel Clarke)
Sarah Jane Smith
Martha Jones
Amy Pond
Sarah Jane Smith, played by Elizabeth Slader, was an investigative
journalist and the longest-lasting companion in the series.  She became
a sort of standard other companions were measured against.
Martha Jones was the first companion noted for her intelligence and
advanced education--and for being in love with the doctor.  Played by
Freema Agyema, Martha was older and more secure than Rose.  She
was a medical student who became a doctor, saving lives and
ultimately the planet.  
Adventures are more exciting with a companion along for the ride.
Amy Pond (Karen Gillan) and Rory Williams (Arthur Darvill) were
the first companions to marry during the series.
Companions regularly come and go on the Dr. Who series.  
Some return home, others find love or purpose on a new
planet.  A few have died.

Recent episodes of Dr. Who have emphasized the toll these
losses take on the doctor.