Imperfect Jr The nightmare reborn.

 in the Cell jrs vs Gohan battle won the smartest grow scared and used tri-form to escape so while Gohan killed 8 out of seven jr.s on dug underground far below the surface and regressed into  an egg. 

where years later the Cell.Jr hatched and entered the imperfect form, as he gave it up when he regressed, So he spent years in the shadows finding the strongest to Absorb while avoiding the Z-fighters  and no doing to much to alert them,  waiting for them to get soft with time before he strikes!