Nick Valentine: Doll. What can he say? He's gotta thing for the classics. (especially you!) He also uses toots on occasion when he's feeling cheeky.
John Hancock: Sunshine or songbird, dollface on occasion. He might call you sugar-lips when flirting with you. He especially likes any nickname that makes you blush and tends to use them in front of others just to see your reaction.
X6-88: Ma'am. No matter how many times you tell him not to call you this, he doesn't listen. If you were to ask him why, he'd say it's out of respect. In actuality, he loves the exasperated expression on your face, it's like gold to him.
Robert MacCready: Knockout, Beautiful, or Honey. You are his sweet, sexy, murderous lover. You remind him of warm honey on a nice summer's day, or a honeybee hard at work providing for others. Alright, maybe he just likes honey, so what. He likes you, he likes honey, it's not that far of a leap.
Preston Garvey: General in the streets, Babe in sheets. He's worried that you two being together would tarnish your reputation, so he insists on calling you General around others. But when you're alone, He's very lovey-dovey. He feels a lot more comfortable being like that.
Deacon: He rarely uses nicknames. He spends the whole day calling you by an alias, so when you're finally alone, he uses your name any chance he gets. Sometimes he just says it just to say it. Although, he does call you Lover occasionally.
Danse: Like Deacon, he's not too big on nicknames. He loves the way your name rolls off his tongue, he practically purrs it when talking to you. He might call you his Soldier whenever you've been injured. It's his way of telling you to be strong.
Piper: Blue or Red. She usually calls you blue, with you being a Vault Dweller and all. But whenever you get flustered or angry, she calls you red because of the way your face flushes. "Lookin' a little red there, Red."
Curie: In her mind, you are a delicate little flower. A soft thing that needs to be nurtured and loved. If you are ever injured, she calls you her dandelion because they always find a way to persevere.
Cait: Darlin'. She loves the way you giggle when she uses that nickname. She likes to whisper it into your neck when you are cuddling, or when you are relaxing on the couch. A small domestic gesture the wasteland has allowed you.
Strong: Strong think nicknames are stupid. Silly human.
Dogmeat: He doesn't know what a "nickname" is, but the way your face lights up when one of your friends uses them makes him happy.