Dexter - the 20-year-old son of recently-introduced character Ava - arrives on Albert Square in January to meet the family he never knew he had.
Best told Inside Soap of his role: "It's especially a pleasure to get to have so many scenes with Ann Mitchell, who plays Dexter's grandma Cora.
"Dexter is really keen to make Cora part of his life, and she warms to him straight away. They're going to have a lovely relationship.
"Ann has really helped me to settle in and encouraged me to be myself on screen instead of worrying about too many other things. I'll take her advice to the grave!"
Dexter quickly makes friends with Abi Branning (Lorna Fitzgerald) upon his arrival in Walford and she invites him to a family dinner, but chaos ensues when his true identity is revealed to everyone.
Best explained: "It's a great introduction for him. There's already a lot of tension within the family for various reasons, so there's a little bit of fun to be had by adding Dexter to the mix. But watch how quickly it all spirals out of control!"
Ava - Cora's long-lost daughter - made her first EastEnders appearance in November.