The Tardis is able to speak due to it having an experimental psychic interface and is able to talk to Max much to his annoyance. He/She is very sarcastic and is always ready to annoy Max, however, He/She does care for Max and wants him to live up to his potential.  However, this doesn't stop him/her being sometimes overly eager to poke fun at Max.

Being a supporting character he/she won't have nearly as many lines as the others but there will be a lot of technical speech so be prepared for that.