The Isolus appeared in the episode “Fear Her” when the 2012 Olympics were taking place and Chloe Webber had been overwhelmed by the Isolus.
The Isolus are compassionate beings in need of love and company. Keeping them going is their need for each other, that urge to find each other can sustain them for thousands of years.
Every Isolus travels in a small pod, this small pod lights up and bleeps when it can sense the love coming from others around it. They use their pod to ride the heat and energy of the solar tides. Keeping themselves amused, they make virtual worlds using Ionic power. You can see an Isolus pod below;
When the Isolus has entered the body of Chloe Webber, Chloe starts acting strangely doing things that an Isolus does. The Isolus have the power to bring drawings to life and change living things into nothing more than a drawing; however the drawings can just about move and change their expression, or give a clue to someone, like The Doctor did by pointing at the Olympic Games Torch, helping Rose figure out what to do in “Fear Her”. Chloe Webber drew the Isolus Mother in the episode (Picture can be seen below).
The Isolus was drawn to Chloe, as she was lonely, just like the Isolus.
At the end of “Fear Her” , The Isolus Pod was charged up by the heat, the love and emotions of The Olympic Torch, The Isolus was sent back to space, and Chloe Webber was back to her normal self, now the Isolus had left her body. There are still many thousands of Isolus out there, so there could always be a return of The Isolus.