Killing Your Darlings

In my book, I've noticed that a particular character--one of the main characters--doesn't seem to do very much. I am currently debating as to what I should do about it. The three options I see before me are these:

• Find some conflict(s) for the character to throw in

--Pros
The character will become more 3-dimensional.
--Cons
I've haven't been able to think of very much despite how long I've known it's been a problem.

• Eliminate the character from the plot entirely

--Pros
It'll mean one fewer character for the reader to worry about.
--Cons
It'll mean a humongous overhaul to the plot. The character is important, but does little themselves. Also, it will change the plot's basic premise significantly.

• Put the character on a bus until I need them.

--Pros
I can still have them do things when I need them to.
--Cons
They will seem to have come out of nowhere when I bring them back. Plus, the plot will still have to undergo a humongous overhaul.

I'm going to have to sit on this one for a while.