For me it's my imagination and what is pleasing to my eyes.Generally personality plays no part,only one character that I created did personality play any part and that was only an experiment.
I'm a big fan of a tabletop game called War Machine, and I've decided to model my human noble after the character High Paladin Vilmon.
I was going to create a dwarf that looks like me in real life (at least face wise as I am 6' 2"), but then I realized the dwarf female characters actually look hot compared to a lot of games. So I'm rolling a cute dwarf who also looks like she can kick butt as a rogue.
For me and my characters? Three of them are from my geek side. Made long ago in a Pen and Paper DnD game that suprizingly survived two evil DM's and one very good DM (came close to getting my human warrior killed by a player fault...remember its best not to toss a halfling thief into the fray of battle unless you really, really dont like that small halfing with a toothpick dagger..I dont think my character slept for a week after that little fight)
Also. The two main characters I have are very personal to me. There names as well. As a thoughtful fun gift some one I knew in a DnD table top game got my charcater names copy wirten do to the length of detail and the time I spent on making them..they only were missing a green card and a SSN hehe