Mamoru is a monkey shikigami born from an egg that Otohiko gave to Nanami Momozono as a test—basically to see if she was ready for the Divine Assembly up in Izumo.
The thing is, Nanami was supposed to hatch the egg over seven days, but she and Tomoe got into it and the egg ended up cracked underfoot by Mizuki. Mamoru started hatching early because of that, but he still came out looking like a tiny monkey despite the rough start.
Nanami kept him around so he could keep growing on her chi. A few days in, though, a yokai attacked her in the library and Tomoe had to bail her out. Being exposed to miasma while her chi was already depleted made Mamoru sick. He really took a hit from that whole situation.
Otohiko called her out afterward, saying she wasn't taking her role as a Shikigami caster seriously enough. He told her to stop churning out talismans like they were nothing and to actually think of her Shikigami as living beings. Then he asked her what power she most wanted, and when Nanami answered "to protect," that became the whole point of Mamoru—his name and his ability both.
Content compiled by AnimeList.moe from publicly available sources.
