Lana Lang (Kristin Kreuk) was Clark’s main love interest for most of Smallville, but she didn’t find out his secret until the end of season 6. Clark (Tom Welling) and Lana had a will-they-won’t-they dynamic for the first two seasons and an on-an-off romantic relationship in the years that followed.
During Smallville season 1, Clark and Lana had secret feelings for each other and a normal friendship that wasn’t complicated by Clark’s powers, secrets, or his Kryptonian heritage. That changed after Clark saved Lana from a tornado in the season 1 finale. Season 2 kicked off with Lana suspecting that Clark was hiding something, as she was convinced that he was responsible for her rescue. Her suspicions remained for several seasons, with Clark constantly using lies and excuses to keep the truth hidden. A few episodes after Lana observed Clark using his powers in secret, Clark finally told her that he was an alien from Krypton in the season 6 finale.
Clark could have told her much earlier, but passed whenever there was an opportunity. Part of this had to do with how Clark was raised. It was important to his parents that he keep his secret safe by sharing it with no one, but the situation with Lana was much more complex than it was with Pete (Sam Jones III) and Chloe (Allison Mack). During his high school years, Clark worried that Lana would hate him if she knew because her parents were killed by the meteor shower that brought him to Earth. Despite Lana’s insistence that she’d understand no matter what, he feared that Lana wouldn’t accept him.
This was the biggest reason for Clark’s lies, but it wasn’t the only one. At the end of season 3, Clark decided he would take the risk and tell Lana everything to keep her from moving to Paris. He changed his mind, though, when Pete told him that he was leaving Smallville because the responsibly of keeping Clark’s secret was too much for him. Because of Pete, Clark developed a concern that knowing the truth came with a heavy burden. He didn’t want to put that on Lana. Then, when she returned from Paris in season 4, Clark had no reason to tell her — at this point, the two weren’t nearly as close as they were in the first three seasons.
When Clark and Lana began a serious romance in season 5, his original problems regarding Lana and his secret from seasons 2 and 3 returned. Once again, Clark was afraid of what she’d think, even though her loyalty and love for Clark were indications that she would understand. Clark got past this in season 5 and did tell her, but this moment was undone by her tragic death. After reversing it with time travel, Clark elected to never be honest with Lana, believing that the truth would put her in danger. Clark maintained this opinion well into season 6, but threw caution to the wind when he realized that she was even less safe with Lex (Michael Rosenbaum).
The scenes they shared in the season 5 episode when she briefly knew he was an alien as well as seasons 7 and 8 prove that Lana was indeed capable of handling the truth and that Clark had worried unnecessarily. Clark actually acknowledged this in a later episode. All of this of course raises the question: what would have happened if Clark had been honest with her years ago? Clark and Lana’s story in Smallville could have ended quite differently if he had.