Shinozaki-kun no Mente Jijou

Shinozaki – an utterly ordinary salaryman who has been suffering from a "pain in the neck" – finds his otherwise-typical life suddenly interrupted by a young woman named Yuuyake.

According to Yuuyake, her magician grandmother had modified Shinozaki’s body to an extreme degree such that he’s chock-full of machinery inside! …All of which require regular maintenance that Yuuyake will apparently be performing from now on.

With that, the slightly offbeat daily life begins, between Shinozaki the salaryman, Yuuyake the magician, and a mysterious creature named Grey!

(Source: Pixiv Comics, translated)

Note: Chapter count includes 43 main chapters, one epilogue, and 14 other extra and bonus chapters.