The 27-year-old actor is dating Michelle Randolph, sister to Colton's girlfriend, Cassie Randolph -- and says the former Bachelor has fit right in with the group after the show.
"Obviously I've gotten to know Cassie because of Michelle, and I knew Cassie before she went on the show. She's still the same girl. She's still a lovely girl," Sulkin told his A Cinderella Story: Christmas Wish co-star, Laura Marano, as the pair interviewed each other for ET on Friday. "Michelle and Cassie are both, their family is amazing."
"Legit, they're a very lovely family, they live by the beach. And then Colton's just sort of fitted in beautifully," he added.
Sulkin says he never finished watching Underwood's season of The Bachelor, and doesn't plan to. After all, he's heard about the fence jump -- "He's a strong boy," he joked -- and knows how the season ends. "I personally won't be watching the next season of The Bachelor. I was very invested obviously because of the Colton and Cassie side of it," he explained.
Underwood and Sulkin have bonded over the past year, and were even spotted on a gym outing together before the former football player's season finale aired in March. Double dates with Michelle and Cassie have now become a regular thing, and sometimes they're just as glamorous as the dates on The Bachelor.
"We have ridden -- no, not in a helicopter -- in a plane together," Sulkin admitted to Marano. "I mean, it wasn't a double date. We were just hanging out... we flew to Coachella together."
If Sulkin has it his way, there'll be many more group outings in the future. "I always said that if I were to have a child with Michelle, I definitely want the child hopefully to take Michelle's looks, because she is absolutely stunning," he shared of his model girlfriend. "But in all fairness ...when you're actually in a relationship with somebody, I find that you obviously get to know them spiritually, mentally, and so my favorite thing aside from Michelle's exterior beauty truly is how kindhearted she is."
"It's her, not me. I just date her. I don't do anything to create who she is. But she is, she's a very special soul, a rare soul," he added.
"[She's] warmhearted, and such a kind and sensitive heart," Sulkin gushed. "So I think aside from the exterior beauty, she's dope."
On screen, Sulkin's A Cinderella Story: Christmas Wish character, Dominic Wintergarden, is romancing Marano's character, Kat Decker. The new movie in the Cinderella Story franchise follows Kat, an aspiring singer, who starts working at a job to help her dreams come true -- and falls for the handsome new Santa.
"A Cinderella Story starring Hilary Duff was a very iconic movie," Marano said, noting it was "intimidating" to join the franchise. "But what I like about our movie is it did kind of feel different."
"It's a part of the Cinderella Story franchise, but it's a Christmas movie, and it's a musical, so it's kind of its own movie and it's own world in that kind of way, which I really appreciated, because I think if it didn't have those different elements, I would be intimidated out of my mind," she expressed. "Because yeah, it's iconic. So I was so happy it had this other level to it."
"You delivered your performance beautifully," Sulkin said.
A Cinderella Story: Christmas Wish is now available on Digital, and will be available on Blu-ray and DVD on Oct. 29 from Warner Bros. Home Entertainment.