When applying for a PhD, is it okay for one letter of recommendation to come from a PhD student? I am currently a Master's student and planning to apply for PhD. Now, I have directly worked with two professors for research projects. Unfortunately, those two are the only professors I have taken more than one course with too. With no other professor I had two or more classes so they do not know me well. I could be able to convince one of them to write letters for me. But surely that's gonna be a canned recommendation like "I taught him this course and he's got an A". Besides as an introvert, I am not memorable at all. I don't drop by during office hours to have small chats or ask many questions in the class.
On the other hand, I have a PhD student in my research group. He doesn't have the credentials or experience, but he can write a more personal and accurate recommendation letter for me. I have worked with him closely and he can give his opinion from both our personal and academic interactions. 

Edit: just to be clear, the two professors I have worked with already agreed to give me LORs. I need a third letter.

Edit 2: I have another option. I have worked as a TA for one semester. The instructor can give me an LOR. But he is neither a tenured professor nor a Ph.D. degree holder himself. So I am not much sure about this option. Is it gonna "hurt" my application?

Edit 3: I collaborated with the PhD student before and we are co-authors of two papers.
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.