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 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.
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. Centurites 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".