In the series' first episode, Addison is made to feel unwelcome by the other doctors. They decide to vote on whether she should be allowed to stay at the practice, but when Addison announces she intends to stay whatever the outcome of their vote, they decide to accept her. Addison is attracted to Pete Wilder, the practice's alternative medicine specialist. When Pete stands her up on a date, Addison decides they should be friends and nothing more, and instead accepts a date with Kevin, a police officer she met through work. In a crossover episode, Addison briefly returns to Seattle Grace to help with a patient whose fetus’s heart is growing outside of its body.
he mother and child both survive the surgery and Addison prepares to leave Seattle and return to L.A., telling Richard that she needed to come back to see how much she liked her new life. She leaves, but not before finding Meredith and advising her not to let Derek get away. In season two, Addison learns from Naomi that the practice is failing financially. She tells Sam against Naomi's wishes, and he launches a hostile takeover. Sam's attempt fails but Naomi is made to step down as manager in favour of Addison taking the reins. Feeling betrayed, Naomi breaks off her friendship with Addison. Addison inadvertently prompts the staff to vote between Sam and Naomi, and is surprised when instead, she is elected to lead the practice as its director.
Addison's relationship with Kevin is tested when her brother Archer tells Kevin he is not good enough for her. Addison attempts to reassure him to the contrary, and when Kevin is shot in the line of duty, he moves in with her while he recovers. The two later break up after Addison kisses Dr. Wyatt Lockhart, a well-respected oncologist, with whom she has been competing professionally. In another episode, Addison is surprised when Archer begins working for Pacific Wellcare Center, a rival practice. When he experiences multiple seizures and believes he has a brain tumor, Addison asks Derek to treat him in another crossover episode with Grey's Anatomy.
Derek discovers he has parasites in his brain, and successfully removes them. As the season draws to a close, Dr. Noah Barnes is introduced as St. Ambrose Hospital's cardiothoracic surgeon, with whom Addison shares an attraction. After discovering that her patient is married to him, Addison decides not to pursue a relationship. In season three, Addison continues to lead the practice. Displaying her surgical ability, Addison is able to save colleague Violet Turner and her baby; meanwhile,
Addison's relationship with Naomi seems to be on the mend, despite the latter's departure to Pacific Wellcare. Addison's father, "the Captain" comes for a visit and it's revealed that Addison is aware of how the man has had multiple affairs over the years behind the back of Bizzy, Addison's mother. She's irate when her father sleeps with Violet and her tirade against him causes him to call Bizzy and demand that she tell Addison the truth. She comes and Addison is stunned to see her mother kissing an old friend, Susan. Addison assumes her mother was pushed to it by her father's cheating and is rocked when Bizzy reveals she and Susan have been lovers for twenty years.
She explains she was expected to marry the Captain due to their social positions, and it was only after having children that she realized the truth about herself. Addison is naturally stunned to realize how her parents have been lying to her for her entire life. She tries to handle it well as they leave, falling into her parents' own habit of acting like nothing is wrong. Also during Season 3, Addison and Sam grow closer and later start developing feelings for each other. It was revealed that back in college Sam wanted to ask Addison out, but was too nervous, and he ended up with Naomi. In episode 7 of Season 3, they share a kiss after a terrifying ordeal when they tried to save a married couple who got into a devastating car accident, the husband badly wounded, and the wife pregnant and trapped in the car.
They kiss again when Addison decided to sleep over at Sam's, hurt and vulnerable after finding her mom and Susan kissing. When ex-flame Mark Sloan arrives at the practice with his pregnant teenage daughter, he and Addison have sex in her office, and Sam walks in and finds them. He then shows some jealous tendencies, warning Addison about Mark. Mark and Addison decide not to get back together. Addison and Sam continue to grow close, but she decides to put a halt on their developing relationship in the episode Shotgun, worried about the consequences in her relationship with Naomi for sleeping with Sam. In the episode "Love Bites", Addison revealed that she was in love with Sam and Pete, but she decided to begin a relationship with Pete instead, while Sam was with another doctor, Dr. Vanessa Hoyt.
Despite them being with other people, they still show they harbor feelings for each other, even a hint of jealousy when they see the other with their current significant others. After Sam breaks up with Vanessa, he kisses Addison again, saying he was tired of doing the right thing, and wanted to do the wrong thing. Although she was with Pete, she kisses Sam back and Pete catches them. He then tells her if she wanted to be with Sam, then she should. But she stays with Pete. Naomi then finds out about Sam and Addison's feelings for each other, and she is hurt and ends her friendship with Addison. In the season finale, Addison has to operate on Maya Bennett, her godchild, who got in a car accident on her way to the hospital to give birth. A close friend of hers, Dell later dies of a brain haemorrhage as he was the one driving Maya to St. Ambrose's Hospital. She and Naomi then make up. At the end of the season Addison breaks up with Pete, saying he needed to be with Violet. In the same episode, after some advice from Sheldon, she and Sam finally become a couple.
During the season 4 opener, Sam and Addison's relationship seems to have not worked out, but at the end of the episode they are sharing a bathtub, showing that they are a couple indeed. In the second episode they debate over whether or not they want to go public with their new-found romance, mainly because of Naomi and how she would react.
Addison ends up telling Naomi, and she doesn't take the news well, but Naomi tries to deal with the fact that her best friend and ex-husband are together and maintained a friendship with Addison. Addison and Sam's relationship, which Addison dubbed "AddiSam", is shown to be a healthy and steady relationship, as Addison says that Sam is the perfect guy "with no flaws". Although Addison wants to have children, Sam tells her he's not ready to have more children, as he wants to take time in their romance. Addison's mother Bizzy returns seeking Addison's help in treating her partner Susan, ill with a tumor. After Addison and Dr. Rodriguez seem to have cured her cancer, Addison plans Bizzy and Susan's wedding. After Naomi spots Dr. Rodriguez flirting with Addison, she tells Addison that her and Sam's relationship is becoming serious and real, and that's when Addison usually screws up. Addison assures Naomi that she doesn't want to ruin it with Sam. Out of panic, she proposes to Sam.
When she talks to Naomi about it, Naomi is furious with Addison, as she knew it was too early in the relationship and she knew she did it because she was enticed into flirting with Dr. Rodriguez. After Bizzy and Susan's wedding, Addison tells Sam that she loves him and doesn't want to ruin it with him, and takes the proposal back. During a dance between the two, Susan faints and she is rushed to the hospital. They find out that she has become worse, and the tumor has grown back, and Susan dies. After a heart to heart with her mother Bizzy, Addison finds her mother dead in her hotel room, having committed suicide.
Addison tells The Captain and Archer that their mother committed suicide after the funeral.
She gives Bizzy's eulogy as her mother requested in her will. Sam and Addison end their relationship and Addison goes on a date with a guy named Jake. Ultimately, Addison decides to go away with Jake in the season finale but after watching Violet run away from her troubles and Naomi run towards new prospects, she realized she was running as well and that wasn't her. She turned around and went back to Sam. The season concluded with her telling Sam that she is still going to want a baby.
At the beginning of season 5, Addison runs into Jake again, after she makes an appointment with the fertility doctor in her plight to have a baby. After several unsuccessful attempts at becoming pregnant and a failed adoption, she adopts a baby boy named Henry in episode 5x19. (Wikipedia)