Nii-chan is Shiemi Moriyama's familiar, a baby Greenman spirit she summons during a demonstration after Izumo Kamiki shows off her own spirit foxes. The little guy clearly likes Shiemi and basically lives on top of her head whenever he gets the chance.
In terms of what he actually does, Nii-chan's pretty useful in a fight. He can grow plants from his body—aloe, tree branches, that sort of thing—and uses them to create barriers. He's shown himself to be genuinely protective of Shiemi too; during a mission at an amusement park, he shielded her from falling debris with his plant barriers without hesitation.
Being an infant spirit, Nii-chan doesn't seem to view Shiemi or humans as threats, which probably explains his laid-back attitude around everyone. Communication-wise, he's not exactly verbose—his whole vocabulary is basically just saying "Nii!" in various tones.
Content compiled by AnimeList.moe from publicly available sources.
