Well neither did I.
Consider me spoiled as I haven't finished Pacifist yet, but I'm close.
I probably never will do Genocide because, from what I've seen, is pretty sad compared to Pacifist.
I just really don't have a lot of time to play it because I keep dying on Asgore's fight with NO HEALING ITEMS and I'm too lazy to get gold to buy healing items from MTT-Brand Burger Emporium (because that's the closest shop)
Assuming the player finished Pacifist and started Genocide, Sans would still know the player's name as he knows about different timelines, so he wouldn't refer to Frisk as Chara.