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Yuno's personality is an ambiguous one. For the most part she is a sweet, feminine and sometimes shy girl when around Yukiteru. But, this masks her dark and sanguinary side—a ruthless, cold and calculating psychopath who has little, if any, problem in using others for her ends- her and Yukiteru's survival. She will resort to cruel and usually violent methods, most of the time coming out of nowhere, that generally end in bloody deaths, of either the Diary Holder or any respective follower, effectively making her a Yandere.
Yuno was adopted by her foster parents at a young age, her umbilical cord left to her as a present from her original parents, but was kept at the orphanage. Yuno's life was a happy one for several years until the strain of money began to emotionally and psychologically affect her parents. Yuno's father stayed away at work until the late hours and barely interacted with his family. Yuno's mother began to abuse her, wanting her to become the "perfect citizen", eventually locking her in a cage and timing everything she did, from sleeping to eating. Yuno's mother went to the extreme, leaving Yuno to starve and at times force-feeding her inedible materials, although it is implied in the anime that Yuno did this to herself to survive. While Yuno's father was objective to his wife's severe treatment of Yuno, he did little to nothing to stop her. In the anime, Yuno's mother was ill and believed she was a terrible wife and mother, and believed punishing Yuno was her only way to earn happiness.
(( Work in progress, all of the info belongs to http://futurediary.wikia.com/wiki/Yuno_Gasai. ))