Q is the youngest Holmes brother, and is raised by Mycroft because their parents die when Q is a baby. When Mycroft realizes Q is falling into a bad crowd, following in Sherlock's path of drugs and other illegal activities to get drugs, so he takes Q out of school as a teen and has him 'disappear'. No one but Mycroft knows that the new teen genius being trained to be in the secret service is Q. Q has killed young. Mycroft allows everyone to believe Q ran away or died, depending on how the author wants it. When Q joins MI6 he is believed reformed, and Mycroft tested his loyalty by having him kidnapped and tortured.

-BAMF Q, still a computer genius, but can kick ass
-Sherlock and John stumble across Q while they are working a case and he's on assignment
-Q loves the thrill of a good kill or explosion
-Maybe Q and Bond are 'dating'
-John has seen Q in Afghanistan before he was shot. They worked in the same unit for awhile, and John realizes the teen is a Holmes when Sherlock and him see him on a case.