Puzzles
Neuro likes to play Sudoku. The nearest human equivalent he can think of would be a light snack.
Neuro is always using his demonic tools to help Yako, because really his Sensei/servant/woodlouse is almost completely useless on her own. Their cover would be completely blown in a matter of hours if not for his constant interference.
But using his powers makes Neuro hungry, and they have not yet achieved the popularity/resources to bring in a decent amount of puzzles (though the Information Company headed by servant #2 has helped tremendously). In order to stave off hunger and maintain the energy he needs to solve puzzles, Neuro has resorted to solving various word and number puzzles. While neither contains the evil intent necessary for a true meal, these kinds of puzzles have about the same effect as drinking water. However, water is better than nothing, and Neuro likes to think that solving things like crosswords, while of no real challenge, help him learn more about the world he currently inhabits.
Yako doesn't know that Neuro plays Sudoku while she's at school, partially because he doesn't feel that she needs to know, but mostly because he doesn't really feel like telling her.
That doesn't stop him from stealing her money for new Sudoku books when he runs out.
SnM: So, I've decided that I've adopted Neuro as my sadistic muse. This came about in a rather amusing way. I was bored out of my mind in British Lit, and took all of five minutes to solve the matrix I had been assigned for my 12 tone music homework. When I finished, I decided that I was in a definite mood for more puzzles, possibly a few rounds of Sudoku, and that thought was immediately followed by 'I'm hungry'. So yes, Neuro is in my head, and he wouldn't stop gnawing until I wrote this and another fic.