Alex is the one who barely got here as he once failed his board exams, but he seems to be doing just fine after making it through his internship. He set out on a mission to become a plastic surgeon, but seems to have taken a liking to the neonatals (though he probably would never say it out loud). I think he just works well with women even though he can't manage to handle a relationship. He also took quite a liking to Addison Shepard (Derek's ex-wife) though the romance never really went anywhere. He also had a history of sleeping in on-call rooms with Isobel Stevens for a while. Alex is a little bit of a wild card, he talks back to his superiors, and seems to get himself into trouble a little bit. Although you never thought he could be sweet, he's shown his soft side with patients lately-especially with his Jane Doe from last season as he sat at her bedside often. He lovingly called her "Ava" as she had no memory when she came to Seattle Grace. When her memory did come back, she remembered how boring her life was and liked Alex better. They've had a couple of flings this season when she's dropped in to Grace hoping that he'll whisk her away from her dead-end marriage. Alex seems really guarded still and doesn't want to break up her marriage. He has also managed to sleep with Lexie Grey a couple of times(while seeing Ava), but when he had to fill out a "Sex Survey" in episode 14, he completely forgot about her. In recent episodes we have hints of him having a troubled past, but we have yet to see what that is and what he's been through. Soon "Ava" returns and tells him she's pregnant, but at the same time Izzie, who did her work up in the clinic knows, that Ava isn't pregnant.  By the time she gets permission to tell Alex about it, he's too bitter to take her advise and he thinks he can take care of her on his own.  As things would turn out, Ava is actually sick psychologically.  She suffers from a personality disorder and even though Alex wants to take care of her, he just can't this time.  Through all of this, though, we have learned that Alex took care of his mother, changed her, fed her, and bathed her, when he was young.  Izzie Stevens is the one he confided in.  This gives us a small glimpse as to why he tried so hard to help Ava.  After losing Ava, Alex cries in the arms of Izzie - the only one he's close to. 

In season 5, he turns his back on Izzie once again, as he feels his trust in her may have been betrayed. She won't let him run away from her, though.  She cares too much for him and she knows he's not the jerk he seems.  Alex and Isobel start the relationship over.  Together they are trying to learn how the other works and make it good. Things are looking up for Alex, but Denny is still haunting Izzie.  While Alex can tell that something is wrong with her, he is not sure what to do to help her and she is resisting that  she needs help.

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Season 5:

Izzie's Haunting (episode 8)

Denny's Heart (episode 7)

Learning Army Style (episode 6)

"I Care About You" (episode 5)

Alex and Izzie Fight For the NeuroSurgery (episode 4)

Derek Moves In Roommates Move Out (episode 3) 

Kisses Left Hanging (episode 1 & 2)

Season 4:

Alex Karev Can't Help Ava (episode 16)

Ava Isn't Pregnant (episode 15) 

"Izzie Steven's Secret" (episode 14)

"The 'Sex Survey'" (episode 14)

Ava is Pregnant. . . And Babies are Everywhere (episode 13)

The Contest (episode 12)

Miranda Bailey's Trial of Faith (episode 11)

After All Of Lexie Grey's Hard Work (episode 10)

Lexie Grey and Ava Meet (episode 9)

Lexie Grey Gets Rejected (episode 7)

Meredith, Lexie and Alex Triangle (episode 6)

Halloween at Seattle Grace (episode 5)