Asking for Co-First Authorship
I want to bring this up and ask my PI, but I'm not sure what she'll say. How should I explain my situation to her?

You're thinking about this wrong. It's not the PI's decision. You don't have to convince her, as she has no greater role in establishing authorship than any other, except of course by being the mediator between her own students.
Everyone on the author list has to agree to the ordering (and co-first-authorship). If the present first author agrees you should share first authorship, that's the big hurdle. I would suggest having him email all authors saying something like:

Monolo and I have discussed their contributions, and we would like to submit this paper as co-first-authors.

No one is likely to object, unless you're trying to jump from seventh author to first.