The prosecution has shown that Alexander and Arias often traded sexual phone calls and text messages, and engaged in oral and anal sex. On the day she killed him, Arias posed for graphic sexual photos along with Alexander on his bed in his Mesa, Ariz., home.
The defense has argued that Alexander kept Arias as his "dirty little secret" as he pretended to be a virgin to his friends and family.
Daidone's testimony came on the second day of Arias' defense. The attacks on Alexander's character may be the only way to help convince jurors that Arias, who admitted to killing Alexander after initially denying it, was acting in self-defense and should not be convicted of murder.
One observer, veteran defense lawyer Melvin McDonald, said it was like "swimming up Niagara Falls" because of the evidence amassed by prosecutors.
"What you do, obviously, if you're defending this case, especially when the evidence against you is so compelling, is make a case of self-defense. And to do that, you've got to paint this guy as a bad guy," said McDonald, a former judge and prosecutor who has tried cases against Arias prosecutor Juan Martinez.
The testimony today, McDonald said, has still not proven that Alexander might have threatened or been physically violent toward Arias.
"With this other girl, he's feeding her lies and misleading her, but that doesn't show any inclination toward violence whatsoever," McDonald said.