I think the reason Kiri get's so defensive of Spider when Neytiri points out his otherness is because she also considers herself other and even though she has blue skin like the rest of her family she's never felt more different from them and the slight on Spider also felt like a slight on her because she is different too
She tells Jake as much
She has eyebrows and an extra finger on each hand, her mother was human and she's essentially a clone (though she didn't know it at the time)
How can she not take that personally even though it isn't directed at her













