Being a giant, Diane is several times bigger than a normal human being. She is an extremely fair maiden with purple eyes and medium-length brown hair which is tied up into large twin tails. Since she has the physicality of a teenager, her body is both well-developed and curvaceous. Her facial appearance is very childlike, which often gets guys going for her. Her guild mark is located on her left outer thigh.
Upon first meeting Natsu, Diane had ragged clothing, and her hair came down to the back of her head until Makarov was kind enough to make her a new set. Her earliest clothing resembled what Makarov made for her at a young age. The attire consisted of a short-sleeved one-piece orange suit, boots with five crossed laces that extended almost to her knees, and two finger-less gauntlets of a blue-grey leathery material covered in shiny steel stud - with the right one destroyed after battling Gilthunder as Natsu was showing a newcomer to the Fairy Tail Guild. She also wore a backpack that Erza carried in her travels upon joining Fairy Tail.
Diane is a friendly young woman, and is also very confident in her strength and size, often regarding Erza as weak. She cares a lot for the Salamander and holds strong feelings towards him, often leading her into cuddling him, which he strongly dislikes. Despite this, however, she is very short-tempered and will often throw tantrums if someone makes her extremely upset. Diane is also partially jealous of Lucy, who both wish that they could be like the other in terms of size.
Due to her immense size, Diane can only help her comrades in Fairy Tail by fighting with them in their numerous battles. She has expressed her feelings to be permanently small, which is in contrast to Lucy, who would prefer to be Diane's size in order to beat up Natsu for constantly picking on her. Despite having some hatred towards humans, due to them nearly wiping out her clan, she still fights alongside her Fairy Tail comrades in times of need.
Despite her brave and strong appearance, Diane is an extremely fragile person, as the one thing she hates is being lonely. This is shown when Natsu came to believe that Lisanna had been killed by after Elfman lost control of the Beast Soul spell.
As a young giant, Diane was out hunting with her half-giant sister, Elizabeth, when they somehow got separated and Diane ended up getting lost in the forest. She eventually met up with Natsu Dragneel, who took her to a nearby cave, where they had lunch together.