Mr. Smith is a supposing character in the Doctor Who spinoff Sarah Jane Adventures, and the main antagonist of the season 1 finale Lost Boy, Mr. Smith was through to be a extraterrestrial super-computer, but was revealed to be a living organism called a Xylok (a crystaline species) that embedded into a computer in Sarah Jane's attic to help her protect the Earth from alien threats.
Mr. Smith was among the Xylok in a meteorite which crashed to Earth and became trapped in the surface of he planet for 60 millions years with no chance of escape. Centuries later, a eruption of Krakatoa unleased Mr. Smith as the only Xylok free, a geologist recovers the crystaline entity and give it to her friend Sarah Jane Smith. During the time with Sarah Jane, she discover the crystaline was alive and could communicate with her through her laptop, Mr. Smith explains that he can help her safeguard the Earth and track alien threats. Sarah decides to build the crystaline life-force into a supercomputor and named him "Mr. Smith".
After she created the computer to house him, Mr Smith helped Sarah Jane, and offered her knowledge on many things. His help at this point however, was mainly confined to hacking other computers and displaying information on alien life forms. However, in Revenge of the Slitheen, he momentarily revealed his dark side to Clyde Langer, presumably because he realised he might get help from the Slitheen Family.
The Lost Boy
Eventually, Mr Smith was able to team up with members of the Slitheen family on Earth. He helped two of them pose as Luke Smith's true parents so that they could kidnap him, but despite his efforts to separate Sarah from her allies, his deceptions were exposed when the Slitheens' fake history for Luke had too many holes for Maria and Clyde to accept it. Mr Smith attempted to prevent exposure by trapping Clyde within himself, but Clyde was able to pass on a message that warned others of Mr Smith's true agenda. While K9 kept Mr Smith distracted, Alan Jackson used a computer virus against Mr Smith, which disabled him and wiped his database. As a result, Mr Smith forgot his purpose and asked for instructions. Sarah Jane informed him that his mission was to save the Earth, which he accepted and fulfilled thereafter.
See: Mr. Smith
Mr. Smith was shown to be helpful, loyal, supposing to assist Sarah Jane Smith by giving her database of alien life-forces and new reports of disturbed events, always follow her orders without hesitation, showing to be benevolent entity who seek to protect humanity. However, Mr. Smith's true nature was revealed to be a sinister, manipulate, murderous, arrogant, evil, insane, malevolent entity who wants nothing more, but the survivor of his own race by crashing the moon into Earth which will crack the planet like a egg and released other Xylok that been trapped in the Earth's core for 60 million years. Mr. Smith have no regard of killing billiion of people just to save the Xylok suggesting that his species are better and will do so mush more than a human.