"I think they're so lovely," Rutherford told PEOPLE on Thursday at the debut of designer Rogan Gregory's fashion line for Target at Barney's department store in Beverly Hills.
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This means both fictional families have given their blessing: Penn's TV dad Matthew Settle also gave the couple the thumbs-up.
Rutherford says the couple has benefited by being able experience the phenomenon of Gossip Girl's skyrocketing popularity together as their chemistry gelled.
"To be able to go through this ride and success and to have somebody to do that with and share that experience, I think they're both very fortunate," she says.
The actress also says the cast and crew had no trepidation about the coupling, or fears of on-set fallout if things don't work out between them.
"No, because I think they've known each other for a long time," she says. "They're both so together and whatever happens is their business, and I think they're both darling and very together as people."
Still, Rutherford jokingly admitted she's not always on the inside track of the relationship: "Of course the 'mother' is always the last to know."