Quote Originally Posted by grafrich
Is it an intensifier?
Yes, it is.

Would it sound a bit bookish or even overdramatic if used in spoken russian?
I don't know exactly, but I mean, that would sound a little bit overdramatic. I've never heard it in spoken russian, I think. Or I just can not remember this.

Could 'Лишь Только Одна Она' be translated as 'she is the only one?' Or maybe 'the only one is she/her'?
I think, that it could be translated as second variant.