The whole wood is cast in a dull blue and a thin mist swirls between the trees. "My master has said the glow appeared after the explosion" says the Elven mage's apprentice guiding your party. "Should we find the source of this anomaly he will further my training and I will reward you in turn, as promised." he continues. As you go deeper into the forest you notice that it is entirely silent, and the trees themselves seem almost ethereal. Finally in the center of the woods you see an unnatural clearing, the entire surface seemingly a glass mirror. "What in the nine hells is this?" whispers the mage's apprentice, stepping onto the glass before any of you can react. As he does so the mirror seems to melt and condense into a tear shaped pod that hovers off the ground. The mist that has been surrounding you disappears, the pod now is the only source of the blue glow. Slowly, like petals, the center of the mirrored glowing pod peel away. A blue, featureless, humanoid-shaped figure emerges. The pod, and the glow, disappear. Sound returns to the Forrest. The floor of the clearing is now smooth, soft dirt. Seeing the Elven man first, the blue spectral figure begins to take form. A face of Elven features appears, framed in long faint blue hair; pale white skin, nude but featureless like a child's porcelain doll, replaces the blue glow in the shape of a strong elven woman's body. "We do not have much time." That is all she says before she collapses.

One of my players has left me in charge of creating her character. All she askes is to be a horizon walker ranger. The above is how I introduced her to the party, she joins the level 4 group next week. I've decided she is a refugee from the ethereal plane who has taken the form of an elf (Shader-kai stats and abilities). Her home was over run by an all consuming hord of aberrations. She must find a way to keep them from invading the material plane as well.

The problem, I don't know anything about the lore of the ethereal plane or how it works. Could a race of ghost like people live there, formlessly? People use the plane all the time, could a civilization live there unnoticed? It's a Homebrew so anything's goes but I like to keep things familiar so that players can pick up my lore more quickly. 

Any ideas or suggestions? Where did these ethereal people come from?

You could do an expy of the Githyanki, a civilization living on the corpse of a dead deity floating in the astral plane (which is probably the most metal thing in existence). I don't think there's any lore for the Ethereal Plane in the current edition (because spellplague, thanks, 4th edition), but the Forgotten Realms wiki has an article on its previous incarnations. https://forgottenrealms.fandom.com/wiki/Ethereal_plane

I not, Storm Giant Quintessents could be home-brewed into something that fits with the theme of the Ethereal Plane, or you could simply use the wide array of ghost-like creatures (ghosts and wraiths from monster manual, allips from Mordenkainens, etc.) and mix them together to form a civilization. Or you could just use a mind flayer hive.

I made a community set up shop in the ethereal plane as a refugee village. It was a for a silly one shot I wrote to play with my kids; I just needed a mechanic to let them have an adventure in a decorative Christmas village (they had to be shrunk and cross to the ethereal plane to see the reality tied to the model version accessible from the material plane). But the premise was a group of creatures using magic to make a safe, idyllic place to have a village.

Anyway, that’s what I did to use the Ethereal plane as a home. Not canon by any means, but it’s an idea.