Danny Messer, who came from Slaten Island, is a detective at the New York City Crime Lab. Mac Taylor, the head of CSI: NY and Danny's boss, personally selected Danny despite other people going against this decision. Because of this, Danny tries to live up to Mac's expectations to prove himself to Mac and to the team.
Danny is strongheaded and, dare I say, quite the drama queen rofl. He also has a hard time trusting people, partly because he grew up "in a family under surveillance", if I quote the official CSI: NY website. Still, he has a few close relationships such as what he has with Don Flack, as evident in the episode On the Job, where he goes to Flack for some advice. He also seems to be close to Aiden Burn as seen from their interaction.
He seems to have this sort of father-son relationship with Mac, with Mac both stern and strict in dealing with him yet proud whenever he accomplishes something. He also doesn't have any problems getting along with Stella Bonasera and Dr Sheldon Hawkes. Here's a list of the trivia I know about Danny Messer. I'll add more soon; these are just the things pulled out from the top of my head. BEWARE OF SPOILERS! :)
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• Danny's unabridged (well sort of) name is Daniel Messer.
• Danny was once very involved in baseball, but an unfortunate accident wherein he broke his wrist prevented him from further pursuing this talent.
• He studied in the military academy and graduated top of his class! :)
• He was once involved with the Tanglewood Boys, a New York City gang based on the real-life gang of the same name.
• He seems to like talking to himself a lot when he's examining evidence, lolz~
• He's on Mac's promotion list (but not for long XD)
• He has this habit of disobeying orders. Hmm.