However, he eventually learned that this was not the case but all of his mum's loved ones decided to conceal the truth from him - that his father Graham Foster (Alex McSweeney) had raped Little Mo, leading to Freddie's conception.
Now, Freddie knows the truth about his parentage and grapples with his emotions regarding the secrets of his heritage that his loving mum hid from him.
So, what happened to Little Mo in EastEnders and will she return to the BBC One soap?
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What happened to Little Mo Slater in EastEnders?
Little Mo didn't have the easiest life on EastEnders, and the character is probably best known for being part of a powerful domestic abuse storyline at the hands of her husband Trevor Morgan (Alex Ferns).
The character joined without Trevor in 2000 but was later joined by him and it was soon revealed he was abusive towards her.
Over the course of the plot, Trevor wore Little Mo down so much, that she was a shell of her former self and she was unable to leave him.
On New Year's Eve in 2001, Little Mo finally stood up for herself and hit Trevor repeatedly with an iron, seemingly leaving him for dead. When it was revealed he wasn't, Little Mo was charged with attempted murder but made a deal with Trevor to not press charges.
Trevor was killed in a fire in the Slater house.
Little Mo then moved on with Billy Mitchell (Perry Fenwick), but their relationship broke down when she was raped by Graham Foster (Alex McSweeney).
She found out she was pregnant as a result, giving birth to Freddie, but it was too much for Billy, who struggled to get past seeing the rapist whenever he looked at Freddie.
Little Mo did manage to get justice for what Graham did to her, however, and he was jailed for eight years.
After a failed romance with Alfie Moon (Shane Richie), Little Mo's storylines centred around bringing up Freddie - which had its challenges. She made an enemy of Phil Mitchell (Steve McFadden) after she confronted Ben Mitchell for causing harm to Freddie.
In the meantime, she made a romantic connection with her son's doctor, Dr Oliver Cousins (Tom Ellis), but that also ended in tears when Mo quite literally pushed him too hard and they called time.
How did Little Mo Slater leave EastEnders?
Heartbroken, Little Mo decided enough was enough and planned a new life in Barnstaple with Freddie, but when she reached the station, Oliver asked her to go to Leeds with him.
However, Little Mo insisted it was time for her to flourish herself and not be "Little" Mo anymore, leaving Walford in 2006.
The character has been mentioned numerous times since, including when her sister Kat visited her.
Speaking about her exit at the time to The Mirror, actress Kacey Ainsworth said: "Lots of people thought I'd left then and offers started to come in. That's when I first realised that there is another world out there. If it wasn't for that, I wouldn't have thought about it because I love playing Little Mo so much.
"She is a fantastic character but it was time to move on. I wanted to be an actor from the age of five, but I didn't want to become one just to do the same thing all the time. I would have left earlier - I felt ready to go - but they wanted me to play out a leaving storyline. Now I just need to do something new."
Will Little Mo Slater return to EastEnders?
As far as we know, Kacey Ainsworth is not returning to EastEnders as Little Mo Slater.
No announcement of Ainsworth making a return to the soap has been made, nor have any reports suggested a return is on the cards.
Speaking on Loose Women in February, executive producer Chris Clenshaw was asked about a potential return for the character and he kept the hope alive.
Clenshaw commented: "Freddie is in the show, so there's every chance that Little Mo could pop back to Walford."
Last year, Ainsworth told the Daily Star there were no plans to return in the immediate future, citing her very busy schedule.
"I've done so much since I've left. I'm a regular in an ITV series [Grantchester] and we're on to our seventh series of that [...] I do so much stuff, I'm not sure there's room at the moment," she explained.
Despite this, Ainsworth has regularly interacted with EastEnders fans on social media and even shared stories about Little Mo and Freddie.
Ainsworth even tweeted following scenes where Freddie called his mum.
However, could the show have a surprise guest appearance up their sleeve? We'll have to wait and see.
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EastEnders airs Mondays to Thursdays at 7:30pm on BBC One and from 6am on BBC iPlayer. Visit our dedicated EastEnders page for all the latest news, interviews and spoilers. If you’re looking for more to watch, check out our TV Guide and Streaming Guide.