Who is Bliss from Foundation and Earth and what is their importance?

Bliss is a Gaian, and, as such, part of the entire consciousness of Gaia. Bliss is taken on the quest for Earth against Travize's wishes. He only gives in when Pelorat convinces him that having Bliss along could be valuable. Bliss's own reasons for going are twofold. She loves Pelorat and wants to be with him, but she's also committed to protect Travize. It's Travize who holds the power to decide the very fate of Gaia. Though it's not thoroughly explained, Gaia's survival depends on Travize's agreement to incorporate the rest of humankind into that single consciousness.