Who does お増{ます} designate?


I am currently reading a short Japanese novel targeted at learners of
intermediate levels, entitled 野菊の墓. In this novel, multiple times a character
is named only by お増{ます}。Based on the below examples I believe that this
designates a specific character, probably the family servant/maid but looking
for it in either the jisho dictionary or the goo dictionary did not yield more
results that the typical "increase, multiply" sense of 増す。

Examples (taken verbatim):

> ぼくの家には、母とぼく、兄と兄の妻、家の仕事をするお増{ます}~
>
> ぼくはかばんを一つ持って、民子｛たみこ｝とお増{ます}と一緒に舟の乗り場へ行った。
>
> きっと兄の妻やお増{ます}は、母やぼくたちがいないところで~

Is my understanding correct? What is the exact sense of お増? Is it a common
word? Is it associated with a particular politeness level and/or era?

