If you've been watching Nyarko-san: Another Crawling Chaos, you've probably noticed the faces of random white guys which occasionally show up in the background.
One of them, of course, is H.P. Lovecraft (1890-1937), the creator of Cthulhu, Nyarlathotep and countless other beloved tentacled monsters. The other is Lovecraft's friend August Derleth (1909-1971), the second most inﬂuential creator of the Cthulhu Mythos.
The Cthulhu Mythos is the name for the shared universe created by H.P. Lovecraft, who invented a bunch of weirdly-named monsters and gods and magic tomes and encouraged other authors to also write stories about them. However, Lovecraft didn't care too much about consistency, so his universe was fairly loose. What's certain is that humanity is doomed to go extinct, and there are lots of giant godlike aliens (like Cthulhu) and alien races (like the Shoggoths and the Fungi from Yuggoth), not to mention the ultimate gods Azathoth, Yog-Sothoth, Shub-Niggurath and Nyarlathotep, who could destroy the Earth or even kick Cthulhu's ass easily if they wanted to, but who usually just don't care enough.
After Lovecraft died of poverty and stomach cancer in 1937 from eating too much canned beans and coffee and donuts, his more business-savvy friend August Derleth decided to print the ﬁrst books of H.P. Lovecraft stories and to keep the Cthulhu ﬂame alive. (Derleth was actually the one who invented the term "Cthulhu Mythos"; Lovecraft called it by the less catchy name "Yog-Sothothery.") With Derleth's encouragement, horror fanboys and fangirls soon came up with dozens and dozens of new Cthulhu Mythos gods, like Eihort, Glaaki, Shudde M'ell, Yibb-Tstll and many others. Most of Lovecraft's work is out of copyright now, so anyone can make 'em.)
But Derleth thought that Lovecraft's universe, in which humans are basically bugs, was too depressing, so he changed things up a bit. In Derleth's version of the Cthulhu Mythos, some gods are good and some are evil, and the gods are divided up based on the four Western elements earth, water, air and ﬁre, leading to much rivalry between them! The elemental idea is actually kind of sketchy—there's no evidence for it in Lovecraft's original stories—but that didn't stop Derleth. Derleth even invented a whole category of ﬁre deities, including Cthugha, just so the elemental idea would work. As Lovecraft became more popular, Derleth also started writing his own new stories based on vague ideas H.P. Lovecraft had written down in his notes and signing them "By August Derleth & H.P. Lovecraft," because that's money, honey.
Today, most H.P. Lovecraft fans will happily get in long arguments about whether or not August Derleth's ideas were good or not, and some Cthulhu-based games and stories don't include August Derleth's monsters, but if it weren't for Derleth's publicity skillz, Lovecraft probably wouldn't be nearly as famous. Obviously Mahiro likes Derleth, since he has his portrait on his wall. There are many more Lovecraft & Derleth cameos in Nyarko-san if you can ﬁnd them. Of course, any true Cthulhu Mythos fan would probably have Lovecraft's or Derleth's face around their house in all kinds of places, right?