Rubathon Blue was a companion of the Fifteenth Doctor, briefly replacing the human Ruby Sunday when time was temporarily rewritten accidentally.
Biography
Rubathon was a hatchling of the 57th Hemisphere Hatchlings, something which she was prideful of.
Following Ruby Sunday accidentally stepping on a butterfly in Earth's distant past, the timeline was rewritten more dramatically than the Doctor anticipated due to him having forgotten to switch on the butterfly compensation switch in the TARDIS, altering the planet's evolutionary history. Rubathon Blue took the place of Ruby Sunday in this new timeline.
The Doctor expressed alarm at this, which offended Rubathon, who threatened to kill him if he had made an "incorrect accusation" that she had done something wrong. Following the Doctor's revival of the butterfly, Rubathon's timeline was negated, and Ruby Sunday was restored. The Doctor then made sure to turn on the butterfly compensation switch. (TV: Space Babies [+]Loading...["Space Babies (TV story)"])
Personality
Rubathon was seemingly narcissistic, believing herself to do no wrong. Upon the Doctor's exclamation of shock after the butterfly was stepped on, she sternly warned him that she would have to kill him if he were to make such an accusation.
She spoke with a posh accent, and stated her full title of "Rubathon Blue of the 57th Hemisphere Hatchlings" when scolding the Doctor. (TV: Space Babies [+]Loading...["Space Babies (TV story)"])