Grace Tillwell was a Detective with the New Jersey Police Department who was partnered with Detective Danny Williams back in 2001 and who appeared in flashbacks in the Season 3 episode, I Ka Wa Mamua (episode).
Like Danny's partner, Lieutenant Commander Steve McGarret, Grace also preferred driving, even if it was Danny's own car.
While on patrol on September 11th 2001, both Grace and Danny were kidnapped and tortured by two gangsters with Grace later succumbing and dying to her injuries.
Danny, on the other hand managed to escape and kill the two gangsters before fleeing. He later named his young daughter, "Grace" in honor of his deceased partner.