Heidi is a member of the Volturi, a sort of "vampire mafia" that resides in Volterra, Italy. She seems unlikely to be of any use in battle(she is a Meyer-written female after all), so we are forced to assume that she is a part of the Volturi only because the other vampires are too chicken to go out and collect tourists on their own. This, as we are told in New Moon, is a vital function because it is illegal to hunt in Volterra, apparently because losses inside the city would arouse suspicion, whereas, if a person's last postcard said "got invited to a special tour in Volterra today, really excited" and said person was never heard from again, no one would make the connection.
Speshul Vampowerz Edit
Heidi can make people see what they want. She also has the name of
stupid evil genius a Swiss girl who lives with her Grandfather on the Alps the brilliant Stephenie Meyer's sister, which is not a power, but can cause her foes to laugh at her name, while she attacks. Kinda.
Volturi Money Engine Edit
Not only does Heidi bring in blood for the Volturi Debate Team, she also helps them to earn money. She does this by working at brothels when she's not having hot lesbian sex with Renata (because, really what else would she have to do?). She also does porn shoots. (Again, the books totally aren't sexist, right?)