In Smallville, only 12 characters managed to discover Clark’s alien origins on their own. His Kryptonian heritage was the character’s most closely-guarded secret in the Superman prequel series. Throughout the series, Clark and his allies took great precautions in keeping this knowledge from getting out.
As someone who was constantly using his powers for good, people occasionally seeing him use his powers was unavoidable. Over the course of the series, quite a few – friends and enemies alike – learned about Clark’s powers in Smallville. But those who knew the whole truth – that Clark was an alien – was a much more exclusive club. Over time, he came to trust people like Chloe, Pete, Lana, and the members of the Justice League with his secrets, but there were some in Smallville who didn’t need to be told. Not counting his fellow aliens and people from the future who always knew who Kal-El was, a total of 12 Smallville characters figured out that Clark was a traveler from another planet.
12 Ryan James
The first person outside of the Kent family to find out about his past was Ryan James, the mind-reading child Clark befriended in season 1’s “Stray.” Faking amnesia after being forced to use his telepathic powers to help his criminal stepfather and girlfriend, Ryan stayed with the Kents in their home. During this time, he was puzzled by the fact that he couldn’t read Clark’s mind. Ryan read Martha’s mind and realized that there was something of importance stored in the storm shelter. There, Ryan found Clark’s Kryptonian spaceship and swore to keep his secret. Ryan returned in a season 2 episode, where he died of a brain tumor caused by scientists’ illegal experiments.
11 Roger Nixon
Until its destruction in season 2, the spaceship continued to be one of the greatest threat to Clark’s anonymity in the series. Originally at the request of Lex Luthor, Roger Nixon began spying on Clark and came to find out that the former’s suspicions of Clark were well-founded. After losing Lex’s backing, Nixon became determined to get to the bottom of the Kent mystery on his own and even bugged the Kents’ home. Like Ryan, Nixon figured out that the answers were in the storm shelter. When he found Clark’s spaceship, he decided he was going to share this information with the world, but the tornadoes in the season 1 finale disrupted his plans. In the season 2 premiere, the threat of Clark's secret getting out ended when Lex had to kill Roger to save Jonathan's life.
10 Dr. Frederick Walden
In Smallville season 2, Lex Luthor consulted historian Dr. Frederick Walden while digging into the mystery of the Kawatche Caves. After placing the octagonal key in the cave wall, Dr. Walden was bombarded with energy, which came complete with the knowledge of Clark’s identity. After failing to convince Lex that Clark was an alien from another planet looking to conquer the Earth, Dr. Walden took matters into his own hands and tried to kill Clark himself. An accidental explosion triggered during their fight brought about Dr. Walden’s death.
9 Dr. Virgil Swann
Clark and his parents received a tremendous shock in season 2 when it was learned that a reclusive, billionaire scientist named Dr. Virgil Swann somehow knew Clark’s true identity. Played by iconic Superman actor Christopher Reeve, Dr. Swann investigated the meteor shower and secretly intercepted a message from Jor-El, tipping him off to the existence of an alien visitor who had arrived in a spaceship. When Kryptonian symbols were burned onto Clark’s barn, Dr. Swann looked into the circumstances of Clark’s adoption and came to the correct conclusion that Clark had to be the alien in the spaceship. He shared with him what he had learned about Krypton on the condition that Clark admit he was right. Sometime before his death in season 4, Dr. Swann disclosed this secret to Margot Kidder’s Dr. Bridgette Crosby so that she too could help the Kents.
8 Lionel Luthor
One of the Kents’ worst fears was realized in season 5 when Lionel Luthor figured out who Clark really is. Similar to Lex, Lionel had been aware for years that Clark wasn’t whom he appeared to be, but despite all his financial resources, he struggled to solve the mystery. That changed when one of the Stones of Power activated in Lionel’s hand, thus making him the human avatar of Jor-El. It was explained in season 5 that Jor-El’s consciousness being downloaded into Lionel Luthor’s body left Kryptonian secrets within Lionel’s mind. Because of this, Lionel knew Clark’s identity. Thankfully for the Kents, Lionel was turned into a better person by his connection to Jor-El. From then on, Lionel did what he could to protect Clark.
7 Linda Lake
Clark’s identity being exposed by a journalist nearly happened a second time in Smallville season 6, courtesy of Daily Planet columnist Linda Lake (Tori Spelling). Using her ability to take the shape of water, Lake was adept at eavesdropping on conversations without being noticed. Apparently, this was got she got most of her scoops as a journalist. This was also how she picked up on Clark being an alien. After hearing Clark and Chloe discussing his alien origins, Linda was going to publish an article on it before being stopped by Chloe and seemingly killed by Lana. She returned in season 8, and did publish his secret, but her actions were undone with time travel. The threat she posed was ended for good when she was murdered by David Bloome.
6 Patricia Swann
In a season 7 episode, Clark met Gina Holden's Patricia Swann, the daughter of Reeve's character. Sometime after Dr. Swann's death, Patricia used his research to investigate the Traveler. She followed the clues to Smallville, which led her directly to Lionel Luthor. Lionel captured Clark and put him in a Kryptonite cell just to keep Patricia from finding him, but eventually succumbed to Patricia's demands and agreed to let her meet the Traveler. She hoped to become an ally to Clark as he looked to fulfill his destiny, but was murdered by Lex before that could ever happen.
5 Edward Teague
Smallville season 7 exposed the existence of the Veritas society, a secret cabal of billionaires looking for an alien they called the Traveler. Two of their members, Dr. Swann and Lionel Luthor, eventually learned the truth, with a third – Edward Teague – later finding out as well. The father of Jensen Ackles’ Jason Teague, Edward became a priest at St. Christopher’s Cathedral to guard a piece of a device that could be used to control the Traveler. When Clark came looking for it, Edward figured out that he had to be the Traveler. But like so many others to find out Clark’s secret, Edward didn't live long after finding out the truth. He was killed in the season 7 finale by Lex.
4 Lex Luthor
During their friendship, Clark’s impressive feats of strength and other moments on the show often had Lex convinced that Clark had to have powers of some kind. What he never guessed, though, was that Clark was an alien. It wasn’t until season 7, when their rivalry was in full-swing, that it occurred to Lex that Clark was an alien. After spending nearly a full season in search of the mysterious Traveler, Lex asked Brainiac who it was. To which, Brainiac replied, “Don’t you already know?” This was the final clue Lex needed to realize that the Traveler was Clark all along.
3 Sebastian Kane
In season 8’s “Identity,” Clark, Chloe, and Lois had to contend with Sebastian Kane, a villain-of-the-week telepath who read minds through physical contact. Because of this power, he was able to quietly learn Clark’s true nature during a fight. Chloe, having guessed this had happened (and under the influence of Brainiac), killed Sebastian to keep Clark’s secret from being exposed.
2 Tess Mercer
Through a portion of season 8, exactly what Tess Mercer knew about Clark was one of the many mysteries related to Cassidy Freeman's Smallville character. Since Lex had figured out Clark’s secret before his “death” in the season 7 finale, there was a concern that he might have informed her of the truth. As it turned out, Tess didn’t know he was an alien, but knew enough about Lex’s suspicions of Clark to have some of her own. These, combined with what she read in Lionel Luthor’s journal, allowed her to complete the puzzle and deduce that Clark was the elusive Traveler.
1 Davis Bloome
Davis Bloome, the human side of Doomsday, was unaware of his Kryptonian origin or his connection to Clark for much of Smallville season 8. As the story progressed, Davis learned more and more about his true nature. Clark's part in all of it finally came out in "Eternal" when Davis had a talk with Tess. Believing that Clark had to beat his greatest enemy before his destiny could be fulfilled, Tess revealed that Davis' purpose as Doomsday was to kill Clark. Prior to that moment, Davis was under the false impression that Clark was just one of Chloe's friends, but talking to Tess made him realize that both of them were from Krypton.