i’m a big fan of “slightly more galra than he looks but not in immediately obvious ways” keith so consider:

(for reference: my personal headcanon is that Keith is of Galra descent on his mother’s side and is, like, a quarter-Galra at most since the Galra ancestor was a few generations back, which is why his phenotype is so human? this stuff could all work for half-Galra Keith too though I think) 

• Keith having unusually sharp canines
• (which, sidebar, means there’s a solid chance he was That Kid Who Bites in elementary school)
• keith with night vision just a little too good to be human standard
• keith as maybe mildly colorblind? galra eyes from an evolutionary perspective seem to be optimized for low lighting and colors like red and purple, so Keith could theoretically be slightly blue-blind or green-blind (look at tritanomaly or deuteranomaly on this simulator here) (thank u @cardinalstar for figuring the specifics out for this lmao)
• Keith’s natural lifespan might be longer than a human’s, if he averages out between his human dad and his Galra/part-Galra mom? i’m assuming Galra live longer than us
  
• naturally at around the peak of human stamina and stronger than you’d expect someone his size to be
• Keith is actually pretty heavy for his size? like look at how lance - who is taller than him - is bending over from the wind in that one decontamination scene and keith isn’t 

• he’s just denser than a lot of humans lmao
• re: his ability to seemingly sense quintessence: so @cardinalstar pointed out that all energies have a wavelength, right? so if quintessence’s wavelength is infrared-length or shorter/ultraviolet-length or longer humans wouldn’t be able to perceive it but alteans and galrans theoretically could  
• and if you think about it, frequencies are basically just wavelengths, so galrans and alteans could hear quintessence, but to keith it might be like infrasound where he can barely perceive it but can’t really figure out what he’s perceiving - boom, “weird energy in the desert” 
• even if he’s not thrilled about it, tolerates being alone better than a lot of humans (we’re a pretty hypersocial species as a whole), which is how he didn’t go nuts being stuck in a shack in the desert for a year
• keith may not have the big fluffy galra ears but he secretly can wiggle his human ears better than most people
  
• if galra have tapetum lucidum like cats, keith’s eyes being a little shiny if you look at them from juuust the right angle

Feel free to mention any add-ons you might think of!