78351 was taken as a child and raised in a laboratory where he was continually tested on. His biology consisted of "a pinch of Axon, a bit of Ogron, and a dash of Pyrovile." He grew up in a cage-crib beside 78342 and 78346. 78351 and his friends would whisper to one another until the scientists silenced them, comfort each other when they were scared, and 78351 snuck food to 78346 when he did not have enough. At some point before his twelfth incarnation, the Doctor rescued 78351 and his friends as they were the only surviving test subjects and took them to safety in his TARDIS.