Adam Milligan~

Hotshot: Because he tries his best to look cool around you, yet it usually results in him making a mess.
Nerd: Because of his good grades in school, and because he hates it.

Balthazar~

Theif.
You gave him the nickname Theif because he stole your heart with his pickup lines and cuteness.

Bobby Singer~

Dear.
You're rather old fashioned, and so you gave him the quick nickname dear.

Castiel~

My saving grace.
You're not very creative, so when you once called him your saving grace, the nickname stuck

Crowley~

My King.
He is the king of hell. Anything else would be an insult.

Dean Winchester~

Squirrel, shortstack.
You were slightly taller than him when you first met him because of your high heels, you called him shortstack once then and it stuck.

Gabriel~

Spice. Or Candyman
He's the spice to your life. The excitement that you had been missing for hundreds of years. His love of human candies and sweets is what gave him his other nickname.

Lucifer~

Lucy.
You love teasing him and making him embarrassed in front of other hunters, so you call him Lucy in front of the Winchesters a lot.

Sam Winchester~

Moose, Samsquatch.
He is huge compared to you. Hence, Samsquatch. But when Crowley called him moose, it just stuck.