Daniel was Nikita's fiance when she still worked for Division. Although she hid her double life from him, they fell in love nonetheless until her feelings compromised her work for Division. When they decided to run away together, Division had him killed. Later, it is revealed that Owen in fact killed Daniel as part of his cleaner duty.
Daniel's death is what leads Nikita to turn from Division and get revenge on the organization and Percy.
Daniel Monroe was Nikita Mears's fiancé and supposedly first she ever loved. Nikita had planned to run away from Division to start a family with him.
She met him during an undercover mission and started relationship with him, initially because of the mission. He believed her to be an airline consultant, and thought that the job was too stressful for her - resulting in her asking him to leave everything he knew and run away with her. As there was a deep commitment between them already, this, and Daniel asking her to marry him, made them agree to their plans. Nikita was charmed and thrilled by his possibility.
Unknown to Nikita, Division had secretly installed surveillance systems in Daniel's home - and thus learnt of their plan. Unknown to Nikita, Percy issued a kill order on Daniel. Michael knew of this but did not interfere.
Nikita and Daniel had Daniel's parents lakehouse in the Adirondacks to themselves for the weekend. When Nikita arrived, Daniel wasn't inside, so she walked down to the dock and found Daniel floating face-down in the water, dead. The coroner said he drowned, bu Nikita knew it was Division - and thus set her on her path of revenge.
Daniel was later revealed to have been killed by Owen Elliot, a Division cleaner, under Percy's orders, and made it look like a boating accident. Owen later apologized for his actions, and explained when Daniel died; "It was quick. He didn't feel a thing."
Division set up a pressure senser on his grave to catch Nikita.
Nikita visited his grave in "Pilot" for the first time since his death, knowing that Division will find her there and will come for her and using this opportunity to finally pay her respect. She left her engagement ring there. His gravestone reads "Beloved son" meaning he was well-loved by his parents. The gravestone was heavily damaged by gunfire of Division agents who believed clothed mannequin in front of it to be Nikita.