Clark and Chloe have had a close relationship since the beginning of Smallville. When Jimmy was introduced in Season 6 Chloe was put in the middle of the two.
Chloe had been lusting after Clark since Season 1. On the day she arrived in Smallville she gave him his first kiss, saying, "I know you've been thinking about that all day, so let's get it over with so we can be friends."  Nevertheless, she clearly wanted more from their relationship but only had the courage to admit this when she read a letter telling him the truth about her feelings whilst he slept after he had fallen ill.
Clark cared for Chloe and considered starting a relationship by inviting her to the Spring Formal where they shared a dance. However, his feelings for Lana Lang prevented him from taking it any further. Having given up on her feelings for Clark, she spent the summer of 2002 as an intern at the Daily Planet, where she met Jimmy Olsen and began a whirlwind romance  and lost her virginity to him.
When Chloe returned, it was clear her feelings for Clark still hadn't gone, but Clark was keen to just remain friends. Clark re-evaluated these feelings when Lana was in a relationship with Lex Luthor and the events of Dark Thursday unfolded. Chloe gave him a passionate goodbye kiss, "in case they never saw each other again," and Clark appeared to reciprocate. When they were reunited after General Zod was defeated Clark brought up the kiss, but Chloe had since reunited with Jimmy Olsen so she brushed off any chances of hooking up. Clark was visibly disappointed but agreed.
After Jimmy got a permanent job at the Daily Planet, his relationship with Chloe was quickly re-established. Considering Clark's long standing friendship with Chloe and confused feelings for her, he displays obvious discomfort when he realizes Jimmy spent the night at Chloe's, and while witnessing Chloe and Jimmy kissing. This led Jimmy to believe that Clark didn't like him because Clark is madly in love with her, but Chloe assured him that that wasn't the case. Unbeknownst to Jimmy, Chloe and Clark's relationship remained close due to his secret. However, Jimmy remained simultaneously perplexed and jealous of Chloe's devotion to Clark, even though both Clark and Chloe told him repeatedly that they were just friends. Jimmy was keen to make an effort with Chloe's friends so attempted to resolve the awkwardness between him and Clark, but Clark just looked confused.
The tension between the three of them eventually resulted in Jimmy breaking up with Chloe when Clark was infected with red kryptonite and went on a jealous rampage at Lex and Lana's engagement party. During this rampage Clark also revealed hidden feelings for Chloe but she put it down to the infection. Jimmy was shocked when Chloe defended Clark for his actions and believed that she would rather be with Clark so the couple decided to take a break.
Some time later, Clark and Jimmy were able to develop a friendship independent of Chloe whilst working to locate Lana's stalker. Jimmy told Clark that he wouldn't stand in his way if he wanted to make a go of it with Chloe but Clark reassured Jimmy that he and Chloe are just friends. He encouraged Jimmy to hold onto her if he has the chance. This made Jimmy feel more comfortable around Clark and Chloe was then able to convince Jimmy that she wanted to be with him rather than Clark, which resulted in them reuniting again.
Their relationship was going well until Chloe discovered she was a Meteor freak. Scared of what Jimmy would think of her, she decided to keep it a secret from him only confiding in Clark. This gave Clark an opportunity to be there for Chloe like she had been there for him. When they discovered that there was a GPS tracker in her, Clark burned it out with his Heat vision. When Jimmy discovered Chloe's injury he was very protective of her and angry with Clark for hurting her. With more secrets between Jimmy and Chloe, their relationship gradually became more strained.
Jimmy left town temporarily to go on an assignment. However, Chloe and Jimmy agreed to continue their relationship.
With only Clark as a confidant, Chloe tried to cure her abilities to save her relationship with Jimmy. When this failed, Jimmy was upset that there still seemed to be something that Chloe was holding back from him. Again, the two separated. With Clark in a serious relationship with Lana, Chloe felt more isolated, especially when Clark disappeared.
Chloe bumped into Jimmy at the elevator in the newsroom. Jimmy was reluctant to go in with Chloe at first but she encouraged him to try to get over the uncomfortable atmosphere that had been between them. They ended up getting stuck in the elevator with a bomb. Jimmy was surprised and angry when Chloe tried to call Clark for help, suspecting that nothing will ever change between Clark and Chloe. However, the stress of the near-death situation made Chloe reveal that she was a meteor freak. When Chloe finally confessed her secret, Jimmy was able to understand Chloe's secrecy and after they are rescued they reconcile.
When Pete Ross returned to Smallville, Jimmy was surprised when he thought that Chloe ditched Pete for Clark. He was clearly nervous that one day Chloe will also ditch him for Clark. After this, Jimmy remained suspicious of her relationship with Clark but decided to give their romance another chance. Later, Jimmy asked Chloe if she wanted him more than she wanted Clark. Chloe didn't answer, but kissed him back. Jimmy seemed content with this response.
Brainiac attacked Chloe, which put her in a coma. This made Clark feel guilty for getting his friend involved in his dangerous life and get hurt in the process. Nevertheless, Chloe recovered when Brainiac was defeated. After nearly losing Chloe, Jimmy proposed to her. However, she was arrested before she could respond.
Clark rescued Chloe from Black Creek and as soon as she saw Jimmy she agreed to his proposal, signifying that she had completely put her feelings for Clark behind her. However, Jimmy's jealousy of Clark was reawakened when he found Chloe's letter to Clark that she wrote when he was sick. Chloe admitted that Jimmy's jealousy was getting tiring but he reacted angrily, wondering why she had never said those things to him.
This led Jimmy to entertain the thought of starting up a relationship with another woman to make Chloe jealous, but Maxima's kiss proved fatal to humans and he collapsed from an endorphin overload. Fortunately, Clark was able to get him to the hospital in time and Jimmy was saved. Concerned about Jimmy, Clark kept a vigil at Jimmy's bedside until Chloe arrived. When Jimmy woke up he confided in Clark about his concerns that Chloe would never get over her first love. Clark insisted that everyone has to move on and that Chloe did and found him. When Chloe arrived she too confessed that Jimmy is the only man for her. Finally, to put a line under the past, Chloe confessed to Clark how she used to feel about him. She showed him the letter, which he was surprised about but admitted that deep down he suppose he knew. They both agreed to leave it in the past and Chloe continued to plan her wedding.
Chloe and Jimmy threw a party to celebrate their engagement. During the party, Lois caused a scene after she got drunk and publicly revealed her true feelings about the marriage. However, Clark interrupted her speech to announce how happy he was for Chloe. By this point, Clark had clearly got over the awkwardness that he felt at the start of Chloe and Jimmy's relationship and just wanted her to be happy.
When Jimmy and Chloe left the party, a psychotic jeweler abducted them to subject them to a lie detector test to prove their love. He began asking questions about Jimmy's infidelity with Maxima and finally asked if Chloe was in love with anyone else. Clearly nervous that she would say that she was in love with Clark, Jimmy insisted that she should keep quiet. However, the abductor shocked him and repeated the question, to which Chloe replied "no". The lie detector proved she was telling the truth. Jimmy was releaved. Having proved their love for each other, Macy released them. When they woke up in their bed, Chloe said she couldn't have survived with anyone else across from her. This caused Jimmy's insecurity about Clark to disappear.
Whilst investigating the red-blue blur, Jimmy finally worked out Clark's secret. This helped him understand Chloe's relationship with him. However, Clark denied Jimmy's accusations and tried to warn Chloe of Jimmy's suspicions. When Jimmy arrived at the Isis Foundation to discover that Clark had beaten him there, he thought this was proof of Clark's superpowers, as he had been speeding all the way from Smallville. Chloe helped cover for Clark but Jimmy was not convinced and was disappointed that his fiance was still siding with Clark. Finally, Clark managed to convince Jimmy that he was not the red-blue blur with Oliver's help. This situation, however, strained Clark's relationship with Chloe, due to the amount of time she puts in hiding his secret.
Brainiac's attack on Chloe started to effect her memories, as he slowly took over her body. Brainiac began replacing Chloe's memories with Kryptonian code, causing her to forgot Jimmy. This caused him much distress and he showed her photographs of them as a couple to try to help her recognize him. He relied on Clark for help and it soon becomes apparent that she only remembers Clark and Davis Bloome. Jimmy was bewildered as to why she should remember Clark when she had forgotten her own fiance.
Clark was able to cure Chloe with the help of Jor-El, much to Jimmy's relief. However, Clark also took the opportunity to erase her memories of his secret. As much as he appreciates her loyalty and cherishes their close friendship, he did this out of love for Chloe, to allow her to have a closer relationship with Jimmy and protect her from his dangerous life. Nevertheless, this proved to only be temporary.
On the day of the wedding, Clark was obviously really happy for Chloe and Jimmy and was delighted to be able to give Chloe away. However, the reception was interrupted by Doomsday, who seriously injured Jimmy. Despite Chloe busily travelling back and forth between Smallville and Star City where Jimmy was in hospital recooperating, she still made time for Clark.
When Jimmy was transferred back to Metropolis General Hospital, Jimmy awoke to find Davis murdering someone. Chloe didn't believe him and thought it was his medication playing tricks on his mind. This was the final straw for Jimmy. After years of playing second fiddle to Clark, Jimmy was not prepared to go through the same thing with Davis so he decided to split up with Chloe. Despite her efforts to reconcile, Jimmy refused to make contact with her. Chloe understood that there had been something bubbling under the surface for a while  due to her close relationship with Clark.
Chloe was angry when she discovered that Clark had sided with Jimmy in the seperation, as he was suspicious of Davis. His suspicions soon proved to be justified, when Davis was revealed to be Doomsday. Having lost Jimmy, Chloe started to prioritise Clark again, which caused her to sacrifice herself by leaving town with Davis to keep him away from Clark. Clark struggled to understand this and was deeply worried about his friend's safety.
Jimmy, obviously still cared for Chloe, as he helped track her down. This led him to discover Clark's secret, when he arrived at Chloe's last known location and found Clark weakened by Kryptonite. Having proof of Clark's abilities, he was able to understand Clark and Chloe's relationship and, when she was finally tracked down, they reconciled. However, Davis then impaled him on a pole. Before Jimmy died he was able to save Chloe from Davis.
Clark attended Jimmy's funeral from a distance, as he was deeply troubled that decisions he had made had caused his friends death. He was also troubled by the fact that he had caused more pain in Chloe's life and decided to cut all ties with his human life.