As a giant, Diane is several times larger than an ordinary human being. She is a fair maiden, described as "lovely, beautiful" with purple eyes and medium length brown hair which is tied up into large twin tails. She has a particularly well-developed and curvaceous body. Like most of the other Seven Deadly Sins, her facial appearance is relatively childlike and has shown little change over the years. Diane's wanted poster depicts her with light colored hair. Her symbol, which is of the Serpent, is located on her left outer thigh.
When she was still a child, Diane wore ragged clothing until King made her a new set. Her clothes during the earlier part of the series resembles what King made for her, having sewn it herself by using leather derived from Dusk Bison skin. Her earlier attire consisted of a short-sleeved one-piece orange suit, boots with five crossed laces that extend almost up to her knees, and two finger-less gauntlets of a blue-grey leathery material covered in shiny steel stud with the right gauntlet having been destroyed by Gilthunder during the fight in the Forest of White Dreams. She also wore a backpack previously used by Meliodas.
Ten years ago, Diane as a Holy Knight wore a full body orange armor with a unique helmet like the rest of her fellow Deadly Sins.
Diane's current clothes is made by Merlin and is almost the same style as her previous one, except for an inverted triangle cut on the chest, and the skirt. She is also wearing black underwear underneath the skirt.
Diane is a very friendly woman, and is also very confident regarding her abilities, appearing to generally consider Holy Knights as weak. She cares very much about her captain and holds very strong feelings for Meliodas, often cuddling him and becoming very delighted from compliments. Her feelings are strong enough for her to overcome her fears for his sake. However, she is very quick to throw temper tantrums, and often does so upon becoming upset. Diane also gets angry and jealous when Meliodas does perverted things to other girls.
Due to her size, she cannot help Meliodas in any way other than fight for him. She has expressed her strong desire to be smaller, contrary to Elizabeth's desire to be strong. Despite Diane stating that she has no interest in humans, she still selflessly saves people who are in danger, like when the elderly man in Vaizel and Zeal were at risk, Diane risked her life to protect them.
Despite her brave and strong appearance, Diane is a somewhat emotionally fragile person, as shown by her hatred of loneliness. This was portrayed when she was a child and insisted that King stay by her side when she was sick instead of getting herbs to heal her, and when she waited in tears for him to return to her. Diane showed the same kind of feelings towards Elizabeth after she was kidnapped by Vivian.