I believe each guardian adds to your elemental resistance. Each guardian provides different stats to these resistances but if I recall, there isn't a huge difference. As for your birth date, that was just thrown in for fun from what I've seen.

I simply used my real birth date and when choosing a guardian, I looked at each of the Twelve and associated each one with a month of the year. Then I chose the one which corresponded to the month I was born (September).