Lana Lang's (Kristin Kreuk) parents died in the Smallville meteor shower of 1989 when she was a little girl. Afterwards, Lana was adopted by her Aunt Nell (Sarah-Jane Redmond) who lived next door to the Kent's. Clark Kent (Tom Welling) had a crush on Lana throughout high school which later developed into a romantic relationship. Unfortunately, the relationship did not work out due to Clark's need to keep his abilities a secret. He never fully trusted Lana with the secret of his origins and was worried she might reject him if she ever learned the truth. Lana was always able to tell that Clark was keeping something from her and could not deal with his trust issues.
Later, Lana married Lex Luthor (Michael Rosenbaum) in Season 6, a former friend of Clark who had a secret obsession with her. The animosity between Clark and Lex made it difficult for Lana and Clark to maintain their friendship because she trusted her husband. When Lana learned that Lex had contrived her pregnancy in order to convince her to marry him, she faked her own death and escaped from Smallville.
Lana returns to Smallville and a relationship with Clark but her obsession with making Lex pay for his deceit consumes her. She leaves both Smallville and Clark behind again. When she steals Lex's "super-suit" technology in Season 8 and wears it, she becomes strong and invincible. Unfortunately her suit absorbs kryptonite which is harmful to Clark. After diffusing a kryptonite bomb, Lana has to leave Smallville for good this time because she can no longer be around him without hurting him.
"What I don't understand is you're willing to walk into bullets for me, but you won't share what's going on inside. Clark, if you don't open up to the people that love you, you'll always be alone. I can't believe you want to spend your life like that."
"Clark Kent, I swear, if you say "I'm like a sister to you" I'm going back to Lex!"