Unlike most other programs, YC does not have mentors--just partners that spend all their time advising startups (and some part-time partners).
This is really important--most people give terrible startup advice, and people that do nothing but select and advise startups all day are probably the best source of good startup advice.
We split into 4 groups, and each company gets 2-3 "group partners" as their main points of contact. We encourage you to talk to these partners at least once a week, and then other partners whenever you need their specific expertise.