As a painting
Josie was originally a portrait of Lady Josephine, commissioned to capture her beauty. The portrait was so rich with animae particles that it took on its own semblance of life. When Lady Josephine died, the portrait was sold at auction. The Twelfth Doctor bought the living portrait and took it to his house in Wales in the 21st century, where he knew Josie would meet his previous self, believing that the younger Doctor would be better suited to help Josie than he would in his current persona. (COMIC: A Matter of Life and Death)
Meeting the Eighth Doctor
Josie took up residence at the house, and took it upon herself to patch it up to the best of her ability, even rearranging the Doctor's book collection. She worked on her artwork in the house, selling on completed pieces to people in the local area.
Whilst painting one day, the Eighth Doctor showed up at the house and questioned what she was doing there. He went on to explain that it was his house and that he was looking for a copy of Jane Eyre that he had left behind. The Doctor and Josie were interrupted by Mrs Fellowes who informed them that Josie's paintings had come to life and were attacking the village. The Doctor scanned Josie with his sonic screwdriver and discovered that she was coated in animae particles, which due to the reactivation of one of the Doctor's old telepathic circuits had caused the trouble in the village.
The trio left the Doctor's house and found that Josie's paintings were rounding up the townsfolk. The Doctor told Josie to create an ending to the story as he was captured by a pair of Witherkins. Josie then ran back to the house and drew a picture of the Doctor. The picture promptly came to life and destroyed the old telepathic circuit, returning the paintings to their original state. The Doctor invited Josie to travel with him, indicating that their first stop would be Lumin's World. (COMIC: The Pictures of Josephine Day)
Travels with the Doctor
On Lumin's World they found themselves in the middle of a warzone. Josie was hit by a piece of crystal that fell from the sky. The crystal came from a Spherion, and began to grow over her body. The Doctor communicated with the Spherions and discovered that they had no idea their offspring were incubated inside of living beings. The Spherions used their harmonics to shatter the crystals inside Josie's body, freeing her. The Doctor then showed Josie a room inside the TARDIS where she could paint. (COMIC: Music of the Spherions)
Travelling to Briarwood house in 1932, the Doctor and Josie found the house to be attacked by the Nixi. Together they helped Bertie, a young boy living in the house, to fulfil his family's legacy in placing the Nixi and their king into deep sleep for another thousand years. (COMIC: Briarwood)
Josie and the Doctor travelled to a Bakri Resurrection Barge in the far future, where the synthetic bodies created for deceased humans began to grow their own conscious and rebelled against the humans. There Josie encountered Lady Josephine, and she told the Doctor who Josie really was. The Doctor accepted Josie as her friend and helped the synthetic humans overthrow their masters.
The Doctor took Josie back to his house in Wales where Josie recounted the remainder of her story to him. The Doctor was happy to let Josie continue her life and offered for her to travel with him once more. Josie accepted the Doctor's offer to take a trip to Epsilon Eridani for egg and chips. (COMIC: A Matter of Life and Death)
Josie and the Doctor were caught up in the events of the Void seeping out into the universe. The Doctor's earlier selves helped them escape from the Void, where they exited into the Ninth Doctor's TARDIS. They remained behind in Bristol to fend off those possessed by the Type 1 TARDIS within the Void. Ultimately, their memory of the event was lost. (COMIC: The Lost Dimension)
On one occasion, in an alternate timeline created by Rassilon's pact with the Cybermen, (COMIC: Supremacy of the Cybermen) Josie and the Doctor were running back to the TARDIS when they found it covered in Cybermats. Their masters, the Cybermen then surrounded them, shouting "save!". (COMIC: Prologue: The Eighth Doctor) These events were unwritten when Rassilon and the Twelfth Doctor worked together to revert the timeline. (COMIC: Supremacy of the Cybermen)
Josie shares the Doctor's eagerness to solve mysteries. (COMIC: A Matter of Life and Death) She also proved capable of such compassion, as shown when she spoke to the Spherions, letting them know the pain how they'll be no better than the Calaxi colonisers if they go through with genocide (COMIC: Music of the Spherions)