The relationship between Klaus and Alaric Saltzman, they were involved when Klaus took possession of Alaric's body in Know Thy Enemy. They were seen interacting again in The Last Day, when Klaus confronted Damon.
In Know Thy Enemy, Maddox kidnaps Katherine and takes her to a place where she wakes up to find Alaric in a chair with Maddox's hands on Alaric's head, chanting a spell. When Maddox finishes the spell, Alaric gets out of the chair and Katherine realizes that Klaus has just possessed him.
In The Last Dance, Alaric/Klaus teaches the history class at school, under the pretense of being Alaric, and sees Elena, the doppelgänger. He shows up at the 60's decade dance, still possessing Alaric's body, and attempts to kill Bonnie by provoking her into using too much power. She dies.
In Klaus, Greta, Maddox and a few other people (presumably witches) bring out a box while Alaric stands helplessly. His eyes are closed. When he opens his eyes, he sees Katherine and says "Elena?" before collapsing. Klaus then leaves Alaric's body.
In The Last Day, Klaus goes to the bar and sees Alaric and Damon. Klaus thanks Alaric for the use of his body, saying, "Thanks for the loaner, mate."
In All My Children, Ric is at the Mystic Grill with Meredith when Klaus arrives with his brother, Kol. Ric keeps an eye on the Originals, letting the others know when to send Caroline in to make a distraction.
In Heart of Darkness, Stefan tries to get Alaric's darker persona to reveal the location of the last white oak stake, which he had hidden. Klaus arrives, snapping Ric's neck in an attempt to kickstart his darker side by killing him. Ric's alternate personality does reveal itself.
In Before Sunset, after Alaric had been transformed into an Original by Esther in her plan to have him hunt down her children. Ric captures Caroline in an attempt to lure the town's vampires to him. Klaus and the others devise a plan to dessicate Ric with Bonnie's help but they need to make a blood connection. They engage him at the school where Klaus, putting up the best resistance against Ric, manages to shove his fingers into Ric's chest, making the blood connection needed. However, Ric overpowers Klaus, nearly killing with the white oak stake before Elena arrives and Klaus flees with her.
In The Departed, after Klaus had been dessicated by Bonnie's spell, Ric desperately searches for his body, which he knows Elena and the others have. He recruits Jeremy into his scheme, finding that Klaus' body is at a storage facility. Fighting past Rebekah and Damon, Ric opens up Klaus' coffin and stabs him with the white oak stake, setting him ablaze and apparently kiling him, shutting the coffin. It seems he's killed Klaus until it is revealed that Klaus body-jumped into Tyler's body. Ric was killed soon after when Elena passed away.
- Klaus likely possessed Alaric to get the others (Damon, Stefan, Elena, Jeremy, Bonnie, Caroline) off his trail so he would have a chance at kidnapping Elena so he could use her in the ritual and sacrifice her to break the curse.
- Klaus has a soul because his spirit possessed Alaric while his body remained still, lacking Klaus' spirit inside of it.