He was found in episode 18 when Cywren was fighting raiders. Cecil was inside when Cywren launched two mini nukes at the raiders base. When Cywren and Timebomb were looking for supplies, she found that he had been flung out the back of the base. She then took him with her.Cecil specializes in evasive maneuvers.
Cecil has proven to be able to not only survive the harshest elements of the wastelands, but also that he can travel anywhere Crywen goes. Even years later in the Mojave the dreaded gnome still finds way in intervene and halt her travels. This is not to say that what he has endured has not taken a drastic toll on his body. Seemingly in more disrepair with every encounter. On one occasion, Cecil was seen in a form both decayed and a hand of a Deathclaw bonded to him as a new arm. Whether this was done to him by choice is unknown.
After the dreadful betrayal, he joined up with Ben's Abraxos for many other adventures. They both tried their hardest to work against everything that Cywren was trying to defend. They were on Point Lookout; Cecil protecting the evil Professor while Ben's Abraxos was in Desmond's Panic Room. After this incident, Cywren has had a fearful reaction to Ben's Abraxos, now knowing what he could do to her plans. Whenever he/she reared his/her ugly head, Cywren would blast him/her to tiny little pieces. Due to mass production, Ben's Abraxos appeared often. However, Cecil was not mass-produced. The last two times he appeared before Cywren woke up in New Vegas was in The Pitt and Andale. Both times she shot him with her shotgun. After these incidents, Cywren came to the conclusion that Cecil was immortal.
Ben's Abraxos' first appearance in the Mojave Wasteland was in an unnamed shack. Cywren went in, got Dean's Electronics, and left without noticing it. Cecil first appearance in the Mojave Wasteland was in the The Forecaster's den.
Apparently, Ben's Abraxos multiplied, because seven copies of Ben's Abraxos were found in the House Resort bunker.
After becoming Grand champion of the arena,Asylum Weaver met a young Cecil before he became an evil gnome. He was a wood elf who asked Asylum if he follow him. Asylum agreed to this. He took Cecil with him to cloud ruler temple where the portal to paradise but Cecil stayed behind.
After returning, Cecil followed Asylum more until many Oblivion Gates were opened in the center of the Imperial City and Mehrunes Dagon appeared. While fighting off enemies, Cecil then ran away and was not been seen since then. From what is known, Cecil ended up running away from the battle into an Oblivion Gate, which closed before he could escape, trapping him there.
It is unknown if this could be the true cause of Cecil's appearance and demeanor in Fallout Tale and Another Fallout Tale, but from what we know, if he doesn't reappear, then these events are the cause of his gnome appearance.