Jo is the second companion of The Beginner.
Jo was punished by the Judoon because of crimes against time as she had meddled up time.. The execution process sent Jo into the time vortex to die where, luckily, The Beginner saved her (accidently). She tagged along with him to travel. This led to a muddled up Aztec/Mayan/Egyptian era, where she met the Death who had plans to destroy Earth in 2012 in order to restore Gallifrey. Luckily, they both saved the world. (2012, Like Father Like Son, Time Lord Duel).
Following capture by the Rock Kind, Jo was killed by Mardock and taken by the Death. This was in order to get the Beginner to use regeneration energy to 'revive her', but instead she was semi-converted into a Cyberman. With Mardock's help, she killed the Death which caused the destruction of the conversion chamber.
She was able to appear in the Beginner's TARDIS and return the regeneration energy to disappear forever. (Battle of the Beginner)