The Faking It star took to Instagram to celebrate Thorne's 18th birthday

By Aurelie Corinthios
Updated October 08, 2015 11:50 AM
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Courtesy Gregg Sulkin

Bella Thorne and Gregg Sulkin certainly aren’t shy about their love for one another – and his birthday message to the actress is no exception.

“Happy 18th birthday to my beautiful and incredible princess,” Sulkin, 23, wrote on Instagram Thursday, captioning a sweet selfie of him planting a kiss on her cheek. “You are such a wonderful, Kind, generous, loving person and you are insanely important to me. Thank you for being that special person in my life that I always wish I could have and for being the best girlfriend on the planet. You have never failed to make me smile or laugh and I hope this year brings you everything you wish for.”

The smitten Faking It leading man also admitted he wasn’t immune to Thorne’s good looks.

“I’m sure I’ve told you in the past, that personality is everything. However, on top of your charisma and captivating energy, you’re also very very very very good looking,” Sulkin wrote. “As my dad would say; ‘boy, you done good.’ I love you. Lots. #18thBirthday #Iloveyou #mybubba.”

Thorne also took to Instagram to share a picture of herself celebrating the big day with her Midnight Sun costars, including Rob Riggle and Patrick Schwarzenegger, who will star as her romantic interest in the new film.

“So happy to get my first birthday cake from my #MidnightSun family!” she wrote, adding that the movie has “such a special script with the most amazing and beautiful people to work with.”

Sulkin and Thorne were friends for six years before their relationship turned into a romantic one earlier this summer.

A source close to the couple told PEOPLE in August that “they treat each other with a great deal of respect and are really enjoying each other’s company” and often double-date with their pals.

Sulkin is “super traditional and old-school,” according to the source, who says the actor is a “true gentleman” who woos his girlfriend with “flowers and handwritten notes.”

RELATED: Faking It Star Gregg Sulkin Has Never Seen an Episode of Game of Thrones

In September, Sulkin told PEOPLE Now that his girlfriend is essentially his “stylist.”

“I kind of need her help all the time,” Sulkin said. “I love it, [getting] a female’s opinion on it, it helps.”