Three of the characters of the title are humans, and one isn't. I will leave it to you to find out a) what kind of "other" the character is, and b) whether or not he is real. For Etta is eighty-two and is losing her memory. Like many dementia sufferers, the long term memories are those that still stick, and this becomes obvious as the book progresses, with memories of Etta's early life; how she connects with Otto and then Russell, and what happens to this trio.
It isn't a difficult read, but people either like this kind of book (which jumps around in time and also has different character's views) or they don't. When I am going to review a book I go to Amazon to look at the 1 and 2 star scores. It is there that I find that some low scores are from readers who don't like the style a book was written in and therefore don't really read that particular book at all. I mean they read it, i.e. they devour the words on each page; but they don't feel it -like eating a slice of lovely cake but not tasting any sugar.