I think that Saki is meant to be a foil for Haruto (and vice versa). Consider the many ways in which they differ, if not be complete opposites:
1. Haruto views becoming a Space Vampire as a terrible curse, something he doesn't want to befall any of his fellow students.
Saki views becoming a Space Vampire as a tremendous benefit, something to be jealously guarded from others (hence her and Haruto agree to keep secret the existence of numerous Valvraves, albeit for completely opposite reasons
)
2. Haruto fights for others (first for Shouko, later for the student-state in general)
Saki fights for her own personal benefit (primarily fame and attention)
3. Haruto doesn't appear to care about becoming more popular, per se.
Saki desires fame and attention.
4. Haruto views personal popularity as a tool that can lead to useful ends.
Saki views personal popularity as a desirable end unto itself.
5. On the whole, Haruto is an optimist that embraces the rest of humanity.
On the whole, Saki is a cynic that views humanity in general in a negative light.
If handled well, you could have a good ying/yang, thesis/antithesis thing going on with these two characters. Hopefully, over time, the two might round each other out. It's not at all hard for me to imagine Haruto and Saki getting into a heated disagreement at some point given their wildly different goals and perspectives.
I think that Saki is a good character, because she's amusing and interesting. But I certainly do see where Ledgem is coming from.
Edit: I'm going to be very interested in seeing if Saki survives through the end of this anime. A character calling oneself "invincible" is about as loud a Death Flag as you can get. Should Saki ever die in combat, I'll definitely point to that as her death flag. Should she survive through the end of this anime, then this will be a rather neat example of a Death Flag being subverted.
