Also, as a tiny bit of explanation (and she'll explain it in character if it gets brought up), Katarina takes the whole 'ship-mates are my family' bit to a little bit of an extreme.  She uses the pain as a reminder of the fact that she has survived as long as she has, even though there are times she shouldn't have made it through.  The ones on her right arm are (for those that aren't aware due to the fact that I kind of put it at the bottom of the sheet) because of the fleet she was born into, which are all gone.