Dr. Samuel "Sam" Beckett

Dr. Samuel Beckett, a scientist, created a machine operated by Ziggy (an AI computer) that lets you to travel back in time. Being impatient to see the results of his work, he entered the chamber and activated it... and then disappeared. What the machine does is makes him jump into someone else's body somewhere in the past (but only in the past from the day he was born to present, which is why his friend Al, could travel to an early period before Sam was born in his one and only adventure during season 4 premiere). Sam can see that person in the mirror and nobody seems him or hears him as Sam, but rather as the person whose body he is inhabiting (and he is not limited by gender and will jump to both male and female characters). During his time in the past, the soul of a person he jumped into is back in the chamber in the present. Now, his only goal is to come back home, and he can only do that by jumping from one person to another. The down side is that he does not remember much about himself (especially the fact that he has a wife waiting for him), and for some reason Al is not supposed to tell him or there could be some complications. Each time he is in some person's body he has to do something good to finish being there and jump to a next body, and with each new jump he hopes to jump back home to present to his own body. But each time he is in someone else's body, he must jump before the time runs out or he could get stranded there in that person's body forever.