A lot of Nitori fanart has her calling the POV character "friend" specifically. What's all that about?
When Reimu defeats Nitori in Mountain of Faith, Nitori gives in on letting her proceed up the mountain, saying humans and kappa are sworn friends (盟友, meiyuu). The term is thus strongly associated with her.
The Process of Affection Level Upgrades"Friend"...A-A human? Awawa...
I-I'm not a bad Kappa.Humans and Kappas are friends you know.
Please let me go.Hey friend, check this out.
It's an automated washing machine.Hehe... Just calling you.I created this based on the outside world's design.
Wanna take a look, friend?