Serana refuses any offers to marry her, based on her dislike of temples and a convoluted history that stretches across 4,000 years. She can be a follower and was programmed by Bethesda with advanced behavior and awareness of her surroundings.
Serana features heavily throughout the Dawnguard expansion, both as an integral part of the storyline and a companion who can be used during the rest of the game. She offers a suite of benefits to the player, including imparting vampirism, crafting unique arrows and being an accomplished necromancer. After the Dawnguard story, she can be convinced to have her vampirism cured.