The Non-Judging Breakfast Club mostly kept to themselves during Gossip Girl's duration. With the sole exception of Dan Humphrey, they didn't allow anyone into their ultra-exclusive group, despite many recurring characters dancing around in the periphery.
These figures came and went, some friends, some foes, and some a bizarre combination of both. All of them, good or bad, added to the scandalous and reprehensible nature that made the show so successful in the first place. And while few of them stayed long enough to become integral parts of the plot, they all left an indelible mark on the show, and some even became fan-favorites.
Vanya
Vanya is the doorman at Serena's building and Dorota's eventual husband. He is a sweet and somewhat unassuming man who cares dearly about his wife. Unlike her, he mostly keeps out of the kids' shenanigans and only becomes involved at her behest.
Vanya makes only a few appearances in the show, and his role is always dependant on Dorota's. However, he makes the most with what little time he gets, and his wedding with Dorota is enough to make him a memorable and beloved character.
Raina Thorpe
Raina is a crucial character in season 4. She is Russell Thorpe's daughter and heiress to his empire. Raina is intelligent and ambitious and an impressive young woman who dazzles those around her. She immediately catches Chuck's eye and dates him for a while, but breaks up with him after seeing how little family means to him.
She also dates Nate, charmed by his sweet and supportive disposition. Raina ends up unintentionally caught in the middle of Chuck's rivalry with her father and, after the confrontation, decides to go back to Chicago. She is one of the few characters smart enough to step back from the UES's damaging influence.
Eva Coupeau
Eva is arguably Gossip Girl's most wholesome character. She rescues a wounded Chuck in Prague and nurses him back to health. The two eventually begin a relationship and settle down in Paris, where she lives. Eva goes back to New York with Chuck, but an angry and jealous Blair succeeds in separating them, and Eva leaves for good.
Unlike many other figures in Gossip Girl, Eva is genuine and sincere. She loves Chuck and is always honest with him. More importantly, she doesn't lose herself when exposed to Chuck's scheming world and instead prefers to walk away.
Ivy Dickens
Like many other characters in the show, Ivy has her ups and downs. At times, she's an intriguing and even menacing character, with the potential of inflicting considerable harm on Serena and company, especially after receiving Cece's inheritance.
Ivy isn't consistent, but neither is the rest of Gossip Girl's main players. Also, and unlike the Georginas and Juliets of the show, she doesn't seem to want to harm anyone. Instead, Ivy just wants a family, but her way of getting one is all sorts of wrong.
Louis Grimaldi
Louis proves to be a wonderfully versatile character. At first, he's the ideal Prince Charming that audiences root for, despite knowing he doesn't stand a chance with Blair. Later, as he becomes more intertwined with the treacherous world of the Upper East Side, he becomes the perfect villain for audiences to hate because of his treatment of Blair.
Louis is a decent character, at first, but Blair just puts him through a lot of unnecessary drama. In the end, it's probably better for everyone that he went back to Monaco where he belongs, and away from the hectic and toxic environment of the Upper East Side.
Diana Payne
Diana is over-the-top and dramatic, but honestly, aren't all Gossip Girl characters? Deliciously played by icy queen Elizabeth Hurley, Diana is a formidable foe for the group during the first half of season 5.
Diana's romance with Nate is more than a little problematic, but her presence does improve the show. She keeps the group on their toes, shares memorable scenes with nearly every member of the cast, and actually poses a threat for them. It also helps that Elizabeth Hurley is clearly having the time of her life and is more than willing to camp things up to an 11.
Jonathan Whitney
Jonathan first arrives in the season 2 episode "Chuck in Real Life." He is Eric's boyfriend for all of season 2 and the first half of season 3. Jonathan is supportive, encouraging, patient, and understanding. He is perhaps the best of the show's boyfriends, mostly because he and Eric stay away from their friends' drama.
After Jenny becomes Queen and Eric becomes obsessed with taking her down, Jonathan breaks up with him, disappointed at his change in attitude. He makes one final appearance in season 4, but after Eric once again mistreats him, he goes away for good.
Juliet Sharp
After Georgina and Jack, Juliet is the most formidable enemy that the group faces. She is cunning and shameless and willing to do everything in her power to bring Serena down, and out of all the foes in the show, she comes the closest to doing it. Juliet succeeds in alienating Serena from her friends and family, damages her reputation, and even gets her kicked out of Columbia.
In the end, though, Juliet agrees to back off, as long as Serena gets her brother released from prison. The fact that she gets away with her plan without receiving any sort of punishment makes her one of the best villains in the show's history.
Jack Bass
And speaking of Jack Bass, Chuck's uncle fits perfectly in Gossip Girl. He's a morally reprehensible and shameless libertine who enjoys torturing his nephew. However, and unlike his older brother, Jack's schemes are usually on the petty side, and he never actually intends for Chuck to get killed or permanently harmed.
Jack also walks a fine line between enemy and ally. He dislikes Chuck, but seems to share an only-I-can-hurt-you appreciation for him. By the show's end, Jack is not only part of the group's inner circle, but he's also in a relationship with Georgina Sparks, effectively becoming the sickest and most toxic pairing in the show.
Cyrus Rose
Every Gossip Girl fan knows that Cyrus Rose is the best parent in the show. He is supportive of Eleanor and Blair and extends his selfless friendship to other characters in the show, like Rufus. He is also the one character who is actually non-judgemental, always willing to support his friends and family.
In the show's universe, he's also a Vietnam veteran, who once saved a comrade under heavy fire, and close friends with Lady Gaga, who wrote her hit single "Poker Face" after playing against him.