Hannigan worked with Leon for the first time during his mission to rescue the President's daughter, Ashley Graham. She occasionally contacted Leon to ask how he was coming along or to give him information. When he entered Ramon Salazar's castle, he unfortunately lost contact with her for several hours because Los Iluminados hijacked their communication line. He could not regain the line until after the Island, which housed the hijacking equipment, blew up. Leon attempted to flirt with Hannigan, telling her she was cute without her glasses and asking for her number when he returned. She remained professional, reminding him that he was still on duty, but with a subtle smile.
Hannigan worked with Leon once again when he is sent to the Eastern Slav Republic in order to investigate the use of B.O.W.S. in a war. She tried to convince him to leave the city as per government orders, but he refused and hung up on her. Contact was not made again until after the incident was already over while Leon was at a hotel, continuing his vacation. She informed him that the president of the country, Svetlana Belikova, had resigned and that the chairmen were being taken out of the country.
Hannigan helped Leon and agent Helena Harper by giving them information on the outbreak and guiding them through Tall Oaks as the two try to find their way out of the town. She also provided them with warning about Derek C. Simmons beginning to become involved with the investigation in the attack. Even after Simmons pronounced Leon and Helena as suspects in the attack and death of President Adam Benford, Hannigan continued to help the two of them in secret. Finding Simmons to be rather suspect, as per Leon and Helena's suggestion, Hannigan placed a tail on him after he departed the operations area and informed the two agents of his location. She would later report both Leon and Helena died with the destruction of Tall Oaks to make it easier for them to track Simmons unnoticed. Later in China, Hannigan helped Leon get in contact with Chris Redfield to notify someone to rescue the captive Jake Muller and Sherry Birkin. After the events in Tatchi and Leon and Helena escaped with evidence against Simmons, Hannigan informed Helena in purpose that the investigation concluded that it would be unfair for her to be punished for Simmons' extortion of her.
From the very start and for a majority of her appearances, Hannigan had a very strict air of business. It wasn't until years later when incidents began to get severely more personal that Hannigan finally became less about business, opting to help Leon and Helena even though they were suspected terrorists by the National Security Advisor, Simmons. In this situation, Hannigan acted upon her own personal feelings for the two agents.
In the workplace, many agents and Hannigan's superiors trust her implicitly to the point that even when she oversteps her boundaries, it is rarely ever questioned. She is described by those she works with as being level-headed and intelligent. Aside from her involvement with Leon, which has had the two of them working for a number of years and developing a deep sense of trust between each other, Hannigan also seemed to have had earned the trust of former U.S. President, Adam Benford. This was made apparent not only by Benford placing Hannigan as operations coordinator when he formed the FOS, but by the fact that Hannigan was also chosen to evaluate agents for possible advancement to the Division of Security Operations on his behalf. She seems to mostly know Helena through her profile and agency records as part of this evaluation process, though believes Helena is a strong-willed individual, a trait that she feels greatly overcomes the agent's rash behavior and moments of misconduct.
Very little is known about her personal life, though a fellow member of the FOS has noted that he heard a telephone conversation of Hannigan arguing with her mother. The conversation seemed to indicate that her parents often tried to contact her while at work and attempt to help her find someone for a potential romantic relationship. However, Hannigan informed her parents that she wanted to put her career first and insisted she did not need their help, telling them to stop bothering her at her job over it.