The actor played author, Richard Castle, who solved cases alongside detective Kate Beckett, who was played by Stana Katic.
While their onscreen chemistry was undeniable, sadly there were rumors of a rift between Nathan and Stana when the show folded after eight seasons.
WATCH: Nathan Fillion looks so different starring in ABC show Castle
ABC claimed budget cuts were to blame for Stana being killed off - a plotline which ultimately didn't happen - and released a statement which read: "Kate Beckett has been a beloved character on our hit series Castle for the past eight years.
"We are grateful for Stana Katic's talent and dedication to the series, and we hope to continue our relationship."
But the show was canceled before a ninth season and an alternative ending was created before both stars moved on to other projects.
Nathan and Stana worked on Castle together for eight seasons
Canadian-born Stana, went on to appear in Sister Cities, The Rendezvous, Lost in Florence, A Call to Spy and The Possession of Hannah Grace. She also starred in Absentia, as Emily Byrne, for three years until 2020.
As well as starring in front of the camera, she has worked behind it too and has a number of producer credits under her belt. Stana founded The Alternative Travel Project and is involved in numerous other charities too.
She's been married to Kris Brkljac since 2015 and secretly welcomed their first child in 2021.
Nathan has gone on to have a successful career on The Rookie
Nathan has always denied there was a rift between him and Stana and addressed her Castle departure in a social media message at the time which read: "Stana has been my partner all this time, and I thank her for creating the character of Beckett who will live on for all of us as one of the greatest police officers on television.
He added: I wish her well and have no doubt she will succeed in everything she pursues. She will be missed."
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