Everyone think Kamu is just Eiko's pet
Don't be silly. Eiko is Kamu's pet. :P
Kamu has even said she has no problem with her dog straying so long as it's back in her yard in the morning, or something similar.
Regardless, Eiko is clearly much more interested in Sensei in a romantic sense, yet still chooses to revolve her life around Kamu. Obviously, once the two students graduate, the three of them will have to more actively decide how their relationship shall work.