Kodi’s body tattoos contain UV reactive and bioluminescent ink, meaning that she glows under certain circumstances/the pigment is very vibrant against her skin. In Waterfall especially, her tattoos almost seem to light up on their own, as if living or breathing, while she moves around. In general, they are very vibrant and seem like they’ve not seen a single day of fading or wear/tear.
Her cheek tattoos also glow, but a lot more faintly - to the point where, unless she’s looking right at you, they almost seem like normal tattoos.
The hearts on her cheek, however, can be seen easily by those individuals (such as sans) who can see SOULs, and seem to give off an eerie hollow feeling - as if her SOUL isn’t quite all there. It may appear cracked or miss-colored in some places, or appear as two souls merged as one. It really depends on the individual who’s looking at it. As far as battle goes, often times one of the hearts (the upright one) peels off of her cheek and works as a visual representation of her SOUL - which is also odd, since human SOULs usually do not come from one’s face, but one’s chest or the equivalent.