This is weird, but I suddenly remembered a character who did this, and since I can't recall which manga this is from for the life of me, it's been bothering me.

This character trait wasn't even like intrinsic to the plot or anything; if I recall correctly, it was just one of her many character quirks. I think she chose to do so because the bed was too soft, she wasn't used to it, etc.. I think she surprised at least several people by doing so.

This is really vague, haha. Sorry.

Appleseed maybe? Not under the bed, but she sleeps on the floor (at least at the start) because she's using to sleeping wherever she can in warzones. Or, in the movie she did, don't remember if that was in the manga as well.

Either way, it's likely to be a female soldier character.

All the girls under the organization in Claymore basically sleep at the back of their sword, because it's way more comfy than the bed according to them. Is this the manga you searching for?

Neither of those are it, but I appreciate the help. I think she was just unused to being in a "civilized" enviroment...
I might just give up, because if I can't remember the plot or anything else about the character, it probably isn't worth the search.

RE: surprising several people - I believe one such situation played out along the lines of: rival sits on bed, hears rustling, girl crawls out from under the bed, humorous situation.

Nino was pretty much doing this in Arakawa Under the Bridge. Although she slept IN the bed, not under it.