2010-2015: Conception and early life
Shortly after she left Marlon for his best friend Paddy Kirk, Rhona found out that she was pregnant with Marlon's baby. Marlon initially refused to be a part of the baby's life and didn't attend Rhona's first scan. In mid-December 2010, Rhona was informed that her baby was likely to have Down syndrome, and the tests the following day confirmed this. Rhona was initially unsure whether or not to keep the baby, but Marlon came round and persuaded her to keep their baby. Rhona, Marlon and Paddy decided that the three of them would bring up the baby together.
On 30th May 2011, Rhona went into labour after being accidentally pushed over by Adam Barton, who was fighting with Aaron Livesy. Marlon accompanied Rhona to the hospital where she gave birth to a baby boy. They decided to name their son Leo, and he was christened in St. Mary's Church with Aaron and Laurel Thomas as his godparents.
Paddy was offered a job in New Zealand in 2012, and Paddy, Rhona and Marlon planned on starting a new life there with baby Leo. However, Marlon changed his mind due to his new relationship with Laurel. Paddy and Rhona still intended to move to New Zealand without Marlon, but he couldn't face losing his son. Marlon took out a court injunction to stop them from taking Leo out of the country. After Paddy and Marlon were arrested for fighting in the street, Rhona took Leo to New Zealand, breaking the injunction in the process. She soon brought Leo back after changing her mind about the move.
In 2013, Rhona became increasingly dependent on prescription drugs after she hurt her back. She began to neglect Leo, so Marlon took Leo away from Rhona until she got clean.
In mid-2014 Marlon's ex-wife, Donna Windsor reappeared in the village with Marlon's daughter April Windsor, Leo's half-sister. Marlon internally refused to be part of her life, but Rhona persuaded him to allow Leo to get to know his sister, as long as they didn't know they were siblings. Once it was discovered Donna was terminally ill, Marlon came round and accepted April as his daughter and later became her primary caregiver when Donna died in suspicious circumstances.
2016-: Rhona and Paddy's split
In early 2016, Rhona and Paddy's marriage broke down after Paddy had an affair with Tess Harris, who later died in a hit-and-run accident. Rhona initially restricted Paddy's access to Leo and Paddy left the village for a job in Germany.
Later in 2016, Leo started hurting April and one of the resulting bruises was found by Emma Barton. This ultimately spiralled out of control when April's babysitter and Marlon's girlfriend Carly Hope was accused of child abuse. April admitted to Marlon that Leo was causing her bruises so he made sure that Leo apologised. Some time later, Paddy returned to the village and began bonding with Leo again.
In April 2017, Leo was best man at Rhona's wedding to Pierce Harris. Leo was left at the wedding reception while back at Smithy Cottage, Rhona and Pierce argued over Paddy giving Rhona a necklace, an argument which ended in tragedy when Pierce raped Rhona. Rhona began to cling to Leo as the trauma of what had happened started to overwhelm her, but Pierce was eventually brought to justice when Rhona reported him and he was sentenced to jail time.
In late 2019, Rhona introduced Leo to her new boyfriend Graham Foster even though Marlon disapproved of the relationship due to Graham's shady past and violent tendencies. As Graham and Rhona continued to be targeted by Graham's ex-wife Kim Tate whilst Rhona was starting to feel excluded at the vets, they decided to move to France and take Leo with them. However, Marlon found out and was furious but they continued their plans in secret. On 20th January 2020, their plans were ruined when Graham was murdered by Pierce who had been released from prison and wanted Rhona back.
- Harry Whittaker, the second child actor to play Leo, died aged 3 on 1st July 2014.
- As Leo has grown, the character has been shown to use the sign language Makaton. A scene in late August 2018 showed Pete Barton, the partner of Leo's mother Rhona, attempting to communicate with Leo in Makaton.