Daniel's relationships including friends, enemies and others he has encountered through out his life.
Dean and Daniel have a working relationship, as well as mutual respect for each other. Over time, Dean begins to see Daniel more as a brother than a friend. Daniel respects Dean and helps him whenever he can. Dean hears about Daniel from Bobby, who said he saved his life went dealing with ghouls. The two do finally meet face to face in "Two And A Half Men". Castiel is sometimes jealousy of their relationship, due to fact that they share the same taste in music, food and Dr. Sexy.
Sam and Daniel have a working relationship along side a friendship. Although Sam's relationship isn't as great as Daniel has with Dean, the two are still friends and respect each other. Sam hears about Daniel from Bobby, who said he saved his life went dealing with ghouls. The two do finally meet face to face in "Two And A Half Men".
Castiel and Daniel having a working relationship as well as a mutual understanding. They are also friends and respect each other. The two met each other, sometime before "Caged Heat", because they already knew each other.
Daniel and Crowley had working relationship, as well as mutual respect for each other. Their relationship began many years ago, he eventually betrayed Crowley by allying himself with the Winchester brothers. Thus their working relationship was nullified as a result of this. In season 7, Daniel reluctantly allies with Crowley at times against the Leviathans, notably when him and Winchester brothers need his blood for the weapon that can kill Leviathans. After Dean kills Dick Roman and disappears with Castiel, Crowley appears and kidnaps Kevin Tran refusing to tell him where Dean and Castiel are. Harry works with Sam and Dean to seal up Hell and kill or forever trap Crowley in season 8 after Dean's return from Purgatory.
Daniel and Meg were allies from time to time, while Sam was soulless, later proposes an alliance with the demon against Crowley in an attempt to kill him and get his soul back. After Crowley is "killed", Sam goes to kill Meg, but she flees again. When protecting Castiel, Sam is reluctant to trust her, but Dean tells him its more like an "enemy of my enemy" situation. Daniel works with Meg to stop Kevin Tran from running off with The Word of God and is a little shocked when angels believe he is in a relationship with her. Her goal is to kill Crowley and not to mess with them. Even ally with her to destroy the Leviathans. After they rescue Meg from Crowley's demons, Daniel once again allies himself with her against Crowley. He works with her to demon-proof the building where the angel tablet is and at her prodding confesses everything that has happened. Meg and Daniel fight side-by-side against Crowley's demons and when Crowley himself shows up, Meg tells Sam and Daniel to get Dean and Castiel while she holds Crowley off. Her actions allow Sam, Dean, Castiel and Daniel to escape and as he watches horrified, Crowley kills Meg.
When Dean returns from Purgatory, he purposefully keeps Benny from Daniel. Daniel first meet him after Dean and Benny take down Benny's old nest. Daniel immediately seems to realize Benny is a vampire and only refrains from attacking because of Dean. Daniel discovers his fate and actions in Purgatory finally earn him his trust and acceptance that Dean buried Benny's remains rather than burning them, he tells Dean he finally understands that Benny is different and to go ahead, and leave the door open to bring him back again in the future.
Daniel has met Gabriel before Sam and Dean did. Daniel discovering his identity and still called him the Trickster.