How would Phantom Quest ever operate without this little guy? If it weren't for Mamoru, Ayaka would probably sleep until noon. And even then, it takes an effort. His usual method for getting Ayaka to wake up is to scatter alarm clocks all over her bedroom. And sometimes, even that doesn't work.

The Shimesu family has been a faithful ally to the Kisaragi family for generations, and Ayaka's relatives sent little Mamoru to care for their estranged kin. Effectively becoming Ayaka's secretary and housekeeper, Mamoru uses his computer to keep track of Phantom Quest's employees, clients, leads...and debts.

(source: Absolute Anime)