Dr. Mark Sloan
Background
Mark was born to an upper-class but emotionally distant family in New York City. At a young age, he befriended Derek Shepherd and became like a second son to the Shepherd family, although he would eventually sleep with at least one of the Shepherd sisters, Nancy. Their friendship lasted throughout childhood and into their adult years, and Mark was Derek's best man at his wedding to Addison Montgomery. Mark attended medical school at Columbia University College of Physicians and Surgeons, completed his residency in Otolaryngology, a fellowship in plastic surgery, and quickly became one of the most highly-regarded plastic surgeons on the East Coast with a thriving private practice. As Derek and Addison's marriage became strained, Mark and Addison grew closer, and eventually Mark slept with Addison, only for Derek to catch them in the act.
After Derek declared his marriage to Addison and friendship with Mark over and left town, Mark and Addison lived together for two months. During this time, Addison became pregnant with Mark's baby. He was initially excited about the pregnancy, but Addison had an abortion amidst rumors of Mark's infidelity, and soon left Mark to follow Derek to Seattle Grace.
Professionally, Mark is regarded as a very successful and competent surgeon. Alex Karev referred to him as the "go-to-guy" for plastics, and Richard Webber proposed that the addition of Mark to Seattle Grace's plastics program would double the revenue of other departments. He is skilled at delicate and complex reconstructive procedures, but also performs cosmetic procedures. Of his personal philosophy, he says: "People don't come to me to fix what's on the outside. They come to me to fix what's on the inside."
Mark portrays himself as a confident and often arrogant man, but admits to Meredith Grey that: "My 400 dollar an hour shrink says ... behind this rugged and confident exterior, I’m self destructive and self loathing to an almost pathological degree." Mark has commitment issues, likely stemming from his emotionally challenged relationship with his parents. His friendship with Derek was his single longest relationship. He has a long history of womanizing and one-night stands, and is somewhat indiscriminate about his partners, admitting to affairs with not only Addison but also his tennis partner's wife. Mark is impatient and shows disdain for the responsibility of training interns, but he is also intelligent and conniving, and not above using others to advance his own agenda.
Season 2 (had not arrived at all in season 1)
Mark first appeared in Seattle shortly after the bomb scare at the hospital with the sole intention of bringing Addison back to New York. During this visit, Meredith, Cristina and Izzie discussed a nickname for Mark. They came up with such names as "McSexy" and "McYummy". Meredith was the one who came up with the name "McSteamy". From that moment on, the interns regularly used the nickname behind Sloan's back. He eventually found out about the nickname when Meredith, who was under the influence of painkillers, called to him "McSteamy! Woo hoo!" to which he replied, "Is that what you're calling me now? McSteamy?" She responded by saying, "Yeah, but I don't think you're supposed to know that." Addison rejected Mark's offer to bring her back to New York and Derek rejected his apology, leaving Mark to return empty-handed to New York.
Season 3
Mark returned to Seattle at Addison's drunken behest, an action she would later refer to as a "transcontinental booty-call." Addison rejected him yet again, but Mark was determined not to give up so easily and after a quick trip back to New York to sublet his apartment and sell his practice, Mark took over the plastics program at Seattle Grace.
He was not warmly welcomed by Addison or Derek, although he desperately wanted to repair his relationships with both. He carried on a brief fling with Callie Torres before she reconciled with George O’Malley, and developed a friendship with Meredith, although he flirted with her before she and Derek resumed their relationship. Mark disliked Seattle, annoyed by the weather and the perceived inadequacies of the city in comparison to Manhattan, and was also discouraged by the continued strife among himself, Derek, and Addison. By this time, he was using his interns to do personal chores for him, such as getting him coffee or picking up his clothes from the drycleaner. Where Dr. Alex Karev was willing to do so in order to get his career as a Plastic Surgeon on track, Dr. Izzie Stevens refused. She did as he asked later, however, she told him that he would not win her respect because he's not worthy of that. Somehow, Mark took notice of these words and allowed Stevens, who officially couldn't be in surgery yet, to be in the OR, something considered as worth her respect. Mark was on the verge of terminating his contract with Seattle Grace and returning to New York when he learned of Richard Webber’s pending retirement. Mark entered the race for the chief position and set himself apart by not following the examples of the other candidates, and his unconventional strategy paid off following his board interview, when his “right-now” plan for the hospital was hailed by the board members and Chief Webber alike.
In addition to work and the race for Chief, Mark’s focus remained on repairing his relationships with Addison and Derek. In an effort to stray from the sexual nature of their past attempts, Mark entered a sixty-day abstinence pact with Addison with the agreement that if at the end of two months’ time they had remained celibate, Addison would agree to give a relationship with Mark another chance. However thirty-two days into the pact, Mark witnessed Addison and Alex leaving an on-call room together, and realized what had happened between them. He was hurt that, after trying so hard himself, Addison would do this to him, so he lied to her about the pact, and said he had cheated on her. While he and Derek were still estranged somewhat at the end of the third season, they had made some progress, occasionally having some fun together, and especially after Mark’s support during Meredith’s drowning, and they were able to remain civil at work.
Season 4
In season four, we begin to see some character growth in Mark. First, he rekindles his friendship with Derek almost completely. In the episode "A Change Is Gonna Come" Mark confesses that his only intention for coming to Seattle was to be best friends with Derek again. This is made easier by the absence of Addison, and the emotional turmoil Derek is in due to his failing relationship with Meredith, making Mark's support indispensable. He also forms a friendship with Callie - who often feels like an outsider at Seattle Grace, much like Mark. Callie enjoys the easy company of Mark, where there is comfortable frankness and no pretentiousness.
Sloan suddenly and unexpectedly finds himself attracted to Dr. Erica Hahn, who he thinks is "sassy", and is not like the majority of people that he's dated. She, however, tells him that he is not attracted to her, per se, but rather her talent, confidence, and the fact that she's not attracted to him. This speech has the opposite intended effect though, as Mark just pursues her with greater passion. She continues to reject his advances, bordering on cruel in some of her responses, but he does not give up. Eventually, in episode 4.11, she throws him a bone and tells him that if they didn't work together she probably would date him, but as they do work together, it's not going to happen.
Another possible romance was that with Callie Torres, with whom he had one-night stand after he arrived in Seattle for the second time. They became friends afterward and Callie reunited with George, with whom she earlier broke up with. After George cheated on her, Callie divorced him and Mark tried to help get over the pain, causing them to grow closer. When Callie shared an unexpected kiss with Erica Hahn in front of Mark, she entered a sexual encounter with Mark to prove she didn't have homosexual feelings towards Erica, and Mark helped her realize she did have feelings.
Season 5
His friendship with Callie is carried over into Season Five, where he guides Callie in her relationship with Hahn. For example, he teaches Callie how to tackle the issue of sex with Hahn, and also comforts Callie when Hahn leaves her.
In Season 5 Mark has also shown increased interaction with Lexie. Friends and co-workers (Meredith, Derek and George) have suspected a potential connection there and suspect the two could be dating or sleeping together. On behalf of Meredith, Derek asks Mark to stay away from Lexie. It was clear for awhile that Mark and Lexie genuinely like each other, but from Mark's reaction to Derek it seems that he didn't think about Lexie sexually until Derek told him not to go there. However, despite Derek's warnings, he allows Lexie to seduce him, and the two sleep together. Mark pays for this in at least one way by suffering a penile fracture while sleeping with her, however there have been indications of deeper feelings on Marks behalf, and seems to want to have a relationship with her, as shown in his speech to Derek in "Before and After." Lexie later breaks up with him when he refuses to come forward with them as a couple. Mark, who usually doesn't care much when a woman breaks up, is now confused and tries to make amends. He eventually refers to himself to Lexie in "An Honest Mistake" as her boyfriend, and tells her he wants her to be his date to Derek and Meredith's wedding. Lexie, though flattered by the idea, still tells him if he doesn't tell people, she won't be his date. Mark makes the decision to tell Derek about him and Lexie. Derek, who was already on the edge or a nervous breakdown due to lots of things happening, couldn't control himself and started to beat Mark up. They eventually were both pretty injured and Mark was treated by Callie at the hospital. Lexie arrived after hearing about the fight and tells Mark he was being stupid. Mark tells her it was worth it and smiles to her.
In the next episode "I will follow you into the Dark", Lexie tries to get Mark to talk to Derek, but Mark, who still has his injuries and can't operate yet, refuses to do so and walks away angry. Later Lexie goes to him and tells him that she knows everybody is against them and their chance of survival is very little, but she wants to try it anyway. She also tells him she's sorry for breaking his hand, his penis and just him. Mark then tells she didn't break him, she's the one who put him back together, and then they walk out together as a real couple.
Source: Wikipedia.org