The Solitract was a consciousness or a form of energy. It came from before the universe and stopped the creation of the universe from being possible by preventing the various elements from coming together. The universe was able to exile the Solitract to the Solitract plane where the consciousness would be alone, forced away from the universe so it would never learn how the universe had come about.
The Solitract was able to create a portal to the universe in Norway, 2018, through an anti-zone, and baited people to stay there in order to learn about the universe because it wanted to be a part of it. To do this, it created an image of Trine and Grace O'Brien. As its universe started collapsing from having too many incompatible life forms, the Solitract sent everyone but the Thirteenth Doctor back through the portal after they rejected it.
With only the Doctor left, the Solitract took the form of its favourite image, a frog. While in this form, it spoke to the Doctor with Grace's voice. However, the Doctor recognised that her presence was still causing the Solitract plane to destabilise which would kill both her and the Solitract unless she returned to her universe. Recognising that the Solitract was just lonely and not evil, the Doctor admitted that she wished she could spend more time getting to know it, calling the Solitract her friend and "the maddest, most beautiful thing I've ever experienced and I haven't even scratched the surface."
The Doctor asked it to release her so they could both live on and the Solitract could "keep on being brilliant by yourself." The Solitract told the Doctor that it missed the universe and everything in it. The Doctor promised that they could be friends forever if the Solitract released her. The Solitract agreed and promised that it would dream of the Doctor, "out there without me." As the Doctor left, she blew the Solitract a goodbye kiss.
After making it back to her world, the Doctor told the others that it wouldn't be returning to the universe. The Doctor expressed regret, as she had only just made a new friend. (TV: It Takes You Away)
Being a living universe, the Solitract demonstrated vast powers. Despite being exiled from N-Space, it could still create a portal between its plane and N-Space. It had virtually omnipotent control over its plane of existence and could recreate environments and living beings such as Trine and Grace O'Brien. These duplicates retained all the memories and personality of the person they copied, however they were little more than extensions of the Solitract's own will.
It could expel a person from its plane back through the portal with a single blast of light emitted from one of its human duplicates if it desired.
The Solitract described its true form as "vast and limitless", and could choose to manifest in any shape or form it desired. Its preferred form seemed to be a small frog that spoke with Grace O'Brien's voice. The Solitract justified this choice by saying this form "pleased it as it once pleased Grace". (TV: It Takes You Away)