Integra and Alucard share a complex relationship. Ever since she was a child, the Nosferatu has served under her commands. Integra is also one of few who can stand up to Alucard and demand his respect.
Seras Victoria is the fledgling of Alucard and is yet another of Integra's agents.
Walter C. Dornez has been the Hellsing butler for quite some time. Integra trusts him deeply and Alucard respects him; as well as the other way(s) around.