The one exception to Caleb's loner rule is Lucas Gottesman. Lucas cares a lot for Hanna, and understands she could not be happy without Caleb. Lucas is the first to find out what Mona did to Caleb's goodbye letter and drives to Arizona to bring Caleb back to Rosewood.
He allows Caleb to crash in his room while attending Rosewood High. Caleb, in return, becomes a loyal friend to Lucas, honoring his commitment to watch "The Goonies" with Lucas instead of hanging out with Hanna one evening. He also helps Lucas out with his pursuing Danielle, Caleb and Hanna go on a double date with Lucas and Danielle because Lucas is feeling nervous.
Although at one point in their friendship, Lucas lost a good deal of the money that Caleb has saved up on a black jack card counting scheme that didn't work out. Lucas was desperate and nearly suicidal as he felt that he betrayed Caleb. Caleb has since forgiven Lucas, and Lucas has sold his comic books to make up some of the money he owed Caleb.
In the second season, their friendship continues to grow. Caleb comes to stay at Lucas's house until he is finished with Rosewood High. Later on, Hanna sets Lucas up with Danielle to make him feel less alone, he becomes extremely nervous about their date so Caleb and Hanna accompany him to the date. When Danielle thinks that Hanna is into Lucas, Caleb helps and Danielle comes to realize that Hanna isn't interested in Lucas in that way and feels less threatened.
Later on, during Caleb's birthday party, Lucas goes missing and that really worries Caleb.
When Lucas returns, he admits that he stole some of Caleb's money and used it to gamble. For a while, their friendship is strained but when Lucas sells his comic books to get the money back, Caleb forgives him and they continue to be buddies.