Box was an intelligent portable computer associated with the Preternatural Research Bureau.
Box was originally given to Liz Shaw by an associate of hers going by "Dr Smith", with the stated purpose of "keeping her on her toes"; she was intended to be a companion on her investigations. However, she did not work and remained inactive for many years. She placed Box in P.R.o.B.e.'s storage hold on the "Dr Smith" shelf. (HOMEVID: When to Die)
The Paul Reynish case Edit
In 2014, Giles found Box and repaired her. She took on Liz's voice and assisted her in locating the chair that could safely kill Paul Reynish and in directing her to save Josie Williams. However, Box broke after Paul's explosive death. (HOMEVID: When to Die)
Behind the scenesEdit
Though no pronouns are used for the character in When to Die's dialogue, Box was voiced by Hazel Burrows in the same voice she used when playing Liz Shaw herself, suggesting that Box is probably a "she".
Although for rights reasons, BBV were unable to make this explicit, the "Doctor Smith" who gave Box to Liz is clearly intended to be the Doctor, at some point after Liz stopped being his companion. As such, the gift of Box mirrors the Doctor's gifting Sarah Jane Smith a K9 unit in A Girl's Best Friend to give her companionship some years after she left the TARDIS.