Lana Lang and Lionel Luthor have had a relationship that has continually changed throughout the series. Whilst they started as virtual strangers, they eventually became family when Lana married Lionel's son, Lex Luthor. Ironically, their relationship deteriorated after this, as he embroiled Lana in his dangerous manipulations.
Obsession with Clark's Secret
When Lionel decided to move to Smallville, he became obsessed with trying to discover Clark Kent's secret and went to extraordinary lengths to get to the bottom of it. As Lana had a close relationship with Clark Kent, Lionel saw an opportunity to use her to gather information and employed Adam Knight to get close to her and report back information about Clark.
Lionel was merely using Lana as a pawn in his manipulative games, which was a Luthor family trait. This ultimately put Lana in danger, when Adam tried to kill her. This was not the first time Lionel had indirectly put Lana in harm's way, as he funded Pete Dinsmore's cloning project to clone Lana's childhood friend, Emily Dinsmore, who also tried to kill Lana. Lana remained ignorant to Lionel manipulating her life and they remained relative strangers.
Search for the Stones of Power
Lionel first met Lana in person when he switched bodies with Clark by using the Crystal of Water. He discovered her affair with Jason Teague and made rude comments to them both. Lana was clearly upset by this, although, as he had switched bodies, Lana believed that this was Clark. When she confronted him, Lionel-as-Clark continued his manipulation of her and forced a kiss on her, which damaged her subsequent relationship with Clark.
Once Lionel was returned to normal, he claimed that the experience had changed him. He began to search for the other Stones of Power, which united him with Lana. Living as a changed man, he was more open and honest with Lana and directed her to China to help her find the Crystal of Air. Remaining unaware of his past manipulations, Lana trusted Lionel when he revealed Jason's lies, explaining that the Teagues were direct descendants of Lana's ancestor's arch rival that had been caught up in a battle to find the stones of power.
Lionel later explained that he knew that Lana was "the chosen one", yet he was unconcerned for her safety when Genevieve Teague forced him into revealing that Lana had one of the stones, despite knowing that this would put Lana in danger. Nevertheless, he was also aware that Isobel's spirit resided in Lana and he warned Genevieve not to underestimate her. Although Lionel was proved to be correct, as Lana was possessed by Isobel and killed Genevieve, he unnecessarily put Lana in danger, as he also had one of the stones, which he kept secret.
Lionel's desire for the stones clearly reverted him back to his old ways, as he used Lana as a bargaining chip with Lex, threatening to turn Lana in to the police for Genevieve's murder unless Lex took the stone away from her. Again, Lionel clearly had no concern for Lana, despite involving her in the search for the stones in the first place. Lionel's stone was activated, which sent him into a coma before he had a chance to carry out his threats.
Lana found Lionel after the second meteor shower whilst he remained in his catatonic state but channeled Jor-El's warnings about the Disciples of Zod. Lana was frightened at first, but still unaware of Lionel's manipulations, Lana had no reason to doubt Lionel so she trusted his information of how to defeat the aliens.
As Lex and Lana began a relationship, Lionel also became closer to Lana. He called Lana to his office to discuss Lex's obsession with the piece of alien technology. Seemingly trying to build up an alliance with Lana, Lionel asked for her help in protecting Lex from himself to make sure he didn't become consumed by his obsession. Lionel suggested that they destroy the technology and, whilst Lana appeared to agree, she went behind Lionel's back. During Lana's brief involvement with the Luthors she had clearly picked up their manipulative tendencies, as she threatened Dr. Groll into coming to her and Lex when he discovered any information about the technology rather than trusting Lionel. Nevertheless, Lana was actually the one being manipulated, as Lionel had orchestrated this to test her loyalty to Lex.
Lana relied on Lionel's help when Lex disappeared, although it was clear she didn't completely trust him. She was right to be wary of Lionel, as he threatened Lana when she reconsidered going through with her marriage to Lex. Lionel warned Lana that if she didn't honor the promise that she had made to Lex he would kill Clark Kent. This aggressive confrontation shocked Lana and caused her to go through with the wedding, despite her increasing doubts. On the surface, Lionel and Lana remained friendly, as he welcomed her into the Luthor family, but Lana harbored deep resentment towards Lionel, especially when she discovered that Lex had orchestrated a fake pregnancy.
Lana eventually confronted Lionel about this. However, she was only courageous enough to do this when Lionel was in a severely weakened state after he was wounded in an explosion. Lana was let in to talk to Lionel, despite the severity of his injuries, by pleading with the doctor and feigning concern for his health. She then used this opportunity to inflict pain on him by pressing on his injuries and knotting his IV to force him to reveal the truth to her. It was clear that she blamed Lionel for being trapped in a marriage to Lex and even threatened to kill him. Lionel confessed that he was protecting Clark, much to Lana's shock.
Lana didn't get the whole story from Lionel and remained suspicious of him. To free herself from the Luthor grip, she tried to incriminate Lionel and Lex by getting her hands on evidence regarding Project Ares. When this led to Lana getting shot, Lionel felt guilty for getting Lana involved and apologized to her as she was taken to hospital. Lionel remained concerned for her when she phoned him to reveal that she was leaving Smallville. He tried to encourage her to not do anything hasty and raced round to meet Lana, but as he arrived, her car exploded with Lana in it.
Lionel was shocked and saddened by this and seemed to feel guilty for involving her in his manipulations. Nevertheless, Lana proved to be just as manipulative, as she had in fact faked her death and employed Marilyn to kidnap Lionel in order for her to get revenge.
As Lionel was held captive in a log cabin for several weeks following the collapse of Reeves Dam, everyone suspected he was dead. When Lana returned to Smallville, she came to check on Lionel and found that he was attempting to escape. She hit him with a shovel and he was returned to the cabin. Lionel's past action had clearly made Lana hell bent on revenge and had no concern for Lionel's well-being, as her hatred for him was evident.
Lionel was kept in terrible conditions, which clearly traumatized him. He was eventually rescued by Lex and he immediately went to confront Lana. Despite the limited evidence, Lionel concluded that Lana was responsible, although she denied it. Lana tried to appear friendly and pleased to see that Lionel was safe, but Lionel was able to see through this and he was the first to recognize the change in Lana's personality, which was a result of Lex and Lionel's treatment of her. Lionel later warned Clark of the change in Lana and explained that she had been responsible for his captivity.
With Lana having destroyed any chance of a reconciliation, Lionel's manipulative ways returned when he needed Lana's help to gather evidence that Lex murdered Grant Gabriel. Lionel tried to appeal to Lana's hatred of Lex at first. When she remained uncooperative, Lionel threatened to damage Lana's relationship with Clark by revealing secrets that she had shared with Bizarro but had kept from Clark. Lana didn't respond well to this and refused to be manipulated by Lionel again. She revealed the truth to Clark and put her hatred for Lex behind her. She then confronted Lionel again and put an end to conflict between them by warning him of the growing darkness within Lex and that Lionel was no exception to being one of his targets. Lionel dismissed this claiming that he knew his son better than Lana.
Lana and Lionel remained civil to each other for Clark's sake and remained united in their concern for Clark, as they discussed the dangers of Kara Kent's amnesia. However, Lana suspected that Lionel was responsible for Clark's disappearance when she found kryptonite electrodes in the barn. When Lana confronted Lionel, he denied the accusations and threw suspicion on to Lex. However, when Lionel's guilt was eventually revealed, Lana's relationship with Lionel was completely destroyed, as she supported Clark's decision to cut him out of their lives following this betrayal. In the end, Lana's warning about Lex to Lionel was valid as Lex pushed him out of his office window, preventing further interatctions.