When applying for a PhD, is it okay for one letter of recommendation to come from a PhD student?
You could do this, though it might be questioned. It would be best if the PhD student has some supervisory function in your group, rather than just a "fellow" student. The letter from the student should make your relationship clear so that someone reading it knows whether they should follow up with you or not. 

But you can also, as an introvert, work to increase your own visibility with the faculty so that this need not recur. Many of them are possibly just about as introverted as you are.