Dr. Callie Torres
Was not in the Season 1
Season 2
Callie is first introduced to the show when George O'Malley dislocates his shoulder after falling down a flight of stairs. She gives him her number, and eventually George calls and asks her out. They begin dating, leading to George discovering that Callie has been living in the hospital basement. She defends herself by explaining it’s convenient for work. The Chief later finds out about Callie’s living arrangement and forces her to move out. She temporarily moves in with George, and is subsequently socially rejected by Izzie and Meredith. Soon after, when George becomes uncomfortable with how fast their relationship is progressing, she moves in the Archfield Hotel. Callie is disliked by Izzie Stevens, George's best friend, and becomes upset by the way George seems to place Izzie and Meredith before her, leading to the couple breaking up at the end of season two.
Season 3
After breaking up with George, Callie goes to Joe's Bar, where she meets plastic surgeon Mark Sloan. The two have a one-night-stand and become friends thereafter. George finds out and is jealous of her fling, but is distracted by his father’s ill-health. Callie supports him through his father’s death, and the two marry in Las Vegas on the spur of the moment. Izzie is highly unsupportive of the marriage, and Callie is pleased when George seemingly chooses her over his friendship with Izzie. However, when Callie confesses to George that she is a wealthy heiress and had kept this hidden from him, George feels betrayed. Combined with this, Callie becomes suspicious that Izzie’s feelings for George are more than platonic. Confused, George confides to Izzie about his relationship woes over alcohol, leading to a drunken sexual encounter between the two.
Callie and George reconcile, but Callie remains unaware of George’s one-night-stand. She becomes increasingly suspicious, and announces to George that she wants them to have a baby together. Shortly thereafter, however, she discovers that George has been unfaithful, ending their marriage (much to the disappointment of George's mother who was very fond of Callie). During a talk with George's mom, it is reveald that Callie is an Atheist. Callie moves out of her hotel room, and into the apartment being rented by Cristina Yang. In episode "Didn't We Almost Have It All?", Callie is named the new chief resident, winning over Miranda Bailey and Sydney Heron.
Season 4
As Season Four began, Callie was overwhelmed with her duties as chief resident and her failing marriage to George. Her personal problems and desire to be performing surgery rather than filling in paperwork interfered with her promotion, and in episode "Physical Attraction, Chemical Reaction", she is demoted, and the position of chief resident is given to Bailey.[7] Callie goes on to form a friendship with the new Head of Cardiothoracic Surgery, Erica Hahn. They become incredibly close over time. She experiences some anxiety when Addison Montgomery returns from Los Angeles and wonders if they are a couple.[8] Callie then embarks on a sexual marathon with Sloan in order to prove her heterosexuality, but he eventually helps her realize she has feelings for Erica, and in the last episode of season four, they share a passionate kiss.
Season 5
During the premiere of Season Five, Erica and Callie awkwardly avoid each other. However, by the end of the episode, they agree to be nervous over their bisexuality together. They begin to joke and loosen up over the matter and it can be assumed that they are now embarking on a lesbian relationship. In fact, in a most recent episode, Callie and Erica began to take on a new level of intimacy. As their romance continued, Erica embraced the idea of being gay much faster than Callie leaving her to panic and begin sleeping with Sloan to discover whether men or women is best for her.
After admitting this mistake to Erica, an argument over Izzie Stevens soon erupts into a conversation about the existence of bisexuality. Erica claims that she no longer knows Callie, and after walking away, leaves her job at Seattle Grace. Callie reacts to the breakup by immersing herself in her work; but when that fails, she ends up breaking down crying in the arms of Mark.
Callie, discouraged about her celibacy, begins complaining to Mark and Lexie when they come to her for advice about their relationship. Pediatric attending Arizona Robbins eventually reassures Callie that she need not be alone, telling her that the "talk" around the hospital she has heard about Callie is good, and that after she stops being upset, people will be lining up for her. When Callie asks for some names, Arizona briefly and tenderly kisses her.[10] However, during An Honest Mistake, when Callie asks her out on a date and mentions that it would be her second relationship with a woman, Arizona declines, saying that since Callie is still in the experimenting stage, it would be like dating a newborn. Later, at Joe's bar, when Callie goes up to Arizona and starts talking about how she does indeed have life experience, it turns out that the woman Arizona was talking to is Julia, her date. In the following episode, Callie spends the whole day hiding from Arizona, until at the end of the day, in the elevator, Arizona starts talking to her like nothing had happened. When Callie tells her that they don't have to be friends, that there are many places in the hospital for Callie to hide, Arizona admits that she had misjudged Callie, having panicked, and proceeds to ask to take her out to dinner. Callie, playing hard to get, says that she will check her schedule. As the elevator doors close, however, she runs back and asks if the next day would be all right. In a later episode, Callie and Arizona are seen dancing in Callie's apartment, and later in the episode, she confides in Arizona about her conflicting feelings of hate for Izzie and the desire that she live. Arizona asks her if she was wishing Izzie would die, and Callie said that she was wishing she would live. This helps Callie get over George and help him support Izzie as her best friend. Callie and Arizona are now together, and when Callie's dad comes to Seattle Grace, Callie introduces Arizona as her girlfriend. Callie's dad wants her to move back to Miami, and delivers her an ultimatum: move back or lose her trust fund. Callie decides to cut him off because he cannot accept her for who she is, and is shown crying in the arms of Arizona.
Source: Wikipedia.org