Who is the Salisbury spy Sergei Skripal?

Former Russian double agent Sergei Skripal is in a critical condition in hospital after an apparent poisoning in Salisbury. He and his daughter were found unconscious on a bench on Sunday. She is also critically ill. The incident has drawn comparisons with the case of Alexander Litvinenko, who was killed with polonium by two Russian men
• Woman in Russian spy mystery is Sergei Skripal's daughter
View the video at https://www.theguardian.com/world/video/2018/mar/06/who-is-the-salisbury-spy-sergei-skripal-video-explainer

Source: Guardian News, https://www.youtube.com/watch?v=oEWDgFKJ4W8