For me, Katniss has sharp facial features, also sharp eyebrows, darkbrown hair... Her eyes piercing through. Not at all "cute", but very fierce. (lol

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At first I imagined Peeta as Butters from South Park, but that kind of killed me until I found The Hunger Games website and saw that picture. Lol, figured Butters with a masculine body wouldn't fit. Heh.
Gale - See Yuki in Akkan Baby, not the personality but the looks. Kind of. Pointy black hair and a thin body, still muscular.
For Rue, I didn't really get the line where it says that she has dark skin, so I always saw her as someone with a round face, hazelnut eyes and brown stripey hair.
Foxface... Red hair, freckles, mischievous gaze. Thin body, olive colored skin.
Cato - a muscular, "stupid" looking person.
Clove - Dark hair but still with the looks of a model.
Glimmer - Lol I actually imagined her sparkling because of her name.