Guy Gardener wrote:
Katie, not Kathryn was her character name in the Fairhaven holonovel.
PATTERSON: You're late.
JANEWAY: Sorry, sir.
PATTERSON: What's the threshold of the H-two molecule?
JANEWAY: Fourteen point seven electron volts.
PATTERSON: Third brightest star in Orion?
JANEWAY: Viewed from where?
JANEWAY: Gamma Orionis, or Bellatrix, if you prefer the original Arabic name.
PATTERSON: Not bad. Now, give me a hug, Katie, that's an order.
JANEWAY: I wasn't expecting a pop quiz.
PATTERSON: Just wanted to make sure all those pips haven't made you forget you're a scientist first.
JANEWAY: How could they? I still have nightmares about your fractal calculus final.
Well, some Admiral no-one from her past could get away with calling her Katie.
The DaVinci hologram would have been talking in Italian unless Janeway instructed it to speak in English, but the Universal translator would have wrestled what he said into English.
Maybe her name was not translated from the original Italian?
Holograms still wouldn't be "friends" or "family", it's a video game.
However too answer the question, I figured her family and friends would call her "Katie" 1) as a sign of affection & 2) because they watched her grow up and usually childhood nicknames like "Katie" stick. No matter how old you get, her family and long time close friends would call her by her childhood name.