One underling of Momonga, Narberal Gamma (aka "Nabe"), accompanies him when he starts adventuring in episode 5. I'm guessing he chose her since she looks human and hence can blend in without needing full-body armor or illusion magic, as Momonga does, and as someone like Albedo or Demiurge would.
That's all well and good, but Narberal clearly isn't actually human - she expresses her disgust for humankind on multiple occasions, and it's pretty clear that this isn't self-loathing. What is she?
Narberal gamma is a doppelganger, her normal appearance is different than that of a human. She looks similar to Pandora's Actor. Oddly between a human and a skeleton, but none the less she is able to use her ability to change her form and look like a normal human.
A secondary reason not really mentioned in the Anime and mostly only a supporting reason after the fact, but it is hinted at a few times in the light novel. She is a battle maid, most of the times that Ainz leaves Nazarick or even the new capital of the Sorcerer Kingdom the maids complain saying that Ainz should always have a maid, blah blah blah basically they think he needs a maid to do everything for him because he is a supreme being and he shouldn't have to worry about the little details.
According to Overlord Combat Maid Chibi Comedy Spinoff Episode 8, she's a werewolf, if you haven't watched the series, their three minutes each, so don't just watch the 8th episode, but basically it names off all the different species of the maids, however, the translator might have been a fu~~, I mean, he might have had a terrible sense of humor and decided to put the species names in random order.
