Ensemble Dark Horse / Arrow

• Felicity, Roy and Slade, three recurring characters in the first season, became popular enough to be promoted to series regular for the following season. However, Felicity ended up becoming a major Base-Breaking Character by Season 3 while Slade became the Big Bad of Season Two.
• Tommy Merlyn is popular with the fans due to his charm, chemistry with Oliver, having his own character arc during the first season, and having a tragic death that plays a major role in Oliver's Character Development. Fans had been pushing for him to be brought back on the show, and got their wish in the finale where it's revealed Tommy survived the Undertaking in the new universe and married Laurel.
• He doesn't appear that often and didn't make the cut to series regular, but Walter Steele is nonetheless well-liked by the fanbase.
• Isabel Rochev, but being played by Summer Glau will do that. However, this started to reverse itself with her reveal as Ravager, due to her entire motivation being little more than petty revenge against a dead man.
• Deadshot, for being an Affably Evil Deadpan Snarker. It helps that his comic book counterpart is already incredibly popular. He was popular enough to return as a reoccurring character once he's put on the Suicide Squad. Even after his demise, the writers managed to find ways to bring him back postmortem via his Earth-2 counterpart and hallucination.
• Once he starts making regular appearances, Anatoly Knyazev becomes very difficult to not like. A snarky Russian, former KGB Bratva head with a beard.
• The Dollmaker only appeared in one episode, due to being killed off by Sara/The Canary, but many found him especially memorable due to being quite possibly the most disturbing and loathsome villain in the entire series.
• Clock King was very well-received, for being a Creepy Awesome Evil Counterpart to Felicity and a classic Green Arrow rogue. His Clock King schtick of planning out everything to the second was very well received.
• Felicity's mom Donna has become one, for several reasons. One, her arrival gave us some much-needed backstory and development for Felicity herself (the lack of which was one of the bigger issues people had with the second season). Two, in a single episode she managed to have one great scene after another: a hilarious introduction, a tear-jerking conversation with Felicity, an awesome confrontation with the villain, and a heartwarming reconciliation with her daughter at the end. Three, Charlotte Ross has made it abundantly clear how thrilled she is about being Mama Smoak, and the cast have welcomed her with open arms. She however became very controversial in Season 4 due to Felicity and the Smoak family constantly hogging the screen and contributed to the more controversial aspects of the season.
• Cupid, a Stalker with a Crush who's quite a fun villain among the darker tone of the show, with all of her ridiculous flirtatious lines to Arrow being hilarious.
• When she finally suited up in "This is Your Sword", Tatsu Yamashiro/Katana quickly became a fan favorite.
• Brick. He's a Bane expy and a menace who gives his victims a gun to shoot him before No Selling said bullet, which most would agree is pretty awesome. The fact his act of killing Rebecca Merlyn is what led Arrow to be possible in the first place, can't be overlooked.
• "Haunted" brings back Matt Ryan as John Constantine. It didn't take him long to completely steal the show.
• Tobias Church, a Badass Normal gang leader who unexpectedly becomes a major villain in Season 5. Chad Coleman nails the comic book bad guy performance without going over the top with it, and the writing does a far better job than with Brick of making it believable that this regular guy could give the team so much trouble.
• Rory Regan aka Ragman quickly established himself as one of the highlights of Season 5, remarkable considering that the other members of Team Arrow 2.0 have received some major heat for replacing the likes of Roy, Laurel, and Thea. This is likely owed to being Creepy Awesome and displaying actual competency on missions while having a compelling, likable personality compared to his teammates. As a plus, his issues with Felicity, who was forced to divert nuclear missiles to Havenrock to save Monument Point causing the death of his loved ones in Havenrock the previous season, are addressed and resolved calmly, without any of the excessive drama that tends to plague character relationships on Arrow.
• Vigilante. Even if you don't like his Knight Templar morality, there's few that won't respect him for being an absolute badass, with his borderline superhuman disappearing acts. Then Season 6 fleshed him out and brought him closer to his comics counterparts stance, winning him even more fans. Pity he didn't last long afterwards.
• Stanley Dover was relatively well-liked thanks to being Oliver's only friend during his time in Slabside Penitentiary, but it wasn't until The Reveal that he's a truly insane Serial Killer obsessed with Oliver that he became popular. It helps that Brendan Fletcher really nails the character's absolute insanity. Some even felt that he should have been the Big Bad of the second half of Season 7 instead of Emiko.
• Earth-2 Tommy Merlyn, who became the Dark Archer instead of his father, had all of the charm of Earth-1 Tommy without the assholishness, while badass like Earth-X Tommy but at the same time far more sympathetic.