Diane is a member of the giants clan, and is bigger than everyone that has been shown thus far in the series. Her terrifying strength is perhaps only matched by captain of the seven deadly sins Meliodas and Ban. However, she has shown to overpower even them.
10 years before the series began, Diane was a member of the Seven deadly Sins, a group of criminals accepted into the ranks of the Holy Knights. After an event where they were framed for the death Great Holy Knight, leader of the Holy Knights and the strongest of them, the Seven Deadly Sins split and went into exile.
10 years later, a princess name Elizebath stumbles into the Boar Hat inn, where she meets the leader of the Seven Sins Meliodas, who is the inn keeper. They start on a travel to find the rest of their group to stop the rampaging Holy Knights.
Diane is sleeping in the Forest of White Dreams, where she terrorizes a bunch of prankster imps to guard the forest from Holy Knights. She is exited to be reunited with her captain Meliodas. After a short battle with Gilthunder, the thundery holy knight that travels in a thunder storm, they head to Baste Dungeon. Meliodas is injured and forced to reside in a hospital of sorts, where he is poisoned.
Diane is forced to stay outside and is frustrated about it. Thats when a swarm of bugs sent by one of the Holy Knights attack the village. Diane is initially afraid of them, but overcomes her fear and fight off the bugs, by slamming her fist on the ground.
After eliminating the bugs, she decides to head towards Baste Dungeon by herself, entrusting Meliodas in Elizebaths care. Diane is caught in an illusion cast by the Holy Knight Ruin. Meliodas that follows Diane shortly after, is also caught by the illusion. Diane and Meliodas fight it out, until the illusion is broken by Elizebath. After defeating the Holy Knights Freiser, the three (and pig) goes to Baste Dungeon where they find Ban is already free.
The group now travelled to the Capital of Dead, where they encounter King, another member of the Seven Deadly Sins. Bans memories allow them passage into the Capital of the Dead, a place where spirits of the dead live. A Holy Knight Geera follows them in and starts fighting the seven deadly sins. Geera greatly overpowers them, Diane is left with no choice but to protect Elizabath and Meliodas until King arrived to save the day. Using his sacred treasure, he forced Geera into retreating.
Capital of the Dead soon expelled them. Meliodas, using a marker, draws on Geeras face (she is still temporarily dead at that point), and gives her a hair cut.
After that, they head to Byzel to participate in a tournament where Dianes Sacred Treasure Gideon was being used as a price for winning the competition. Initially, Diane is left out, because giants were not allowed in the tournament. However, an encounter with a mushroom temporarily shrunk Diane and Elizebath. Since Diane was Elizebaths size now, she wore her clothes and decided to take part in the Byzel tournament herself.
Dianes first opponent is the son of the Great Holy Knight for whos death the sins were held responsible. She easily overcame the Knight whos ability was shield projection. In the next round, she fought Hauser, another Holy Knight with the power of tornadoes. Hauser proves a decent opponent as he was able to force Diane to harden her body, turning the color of her skin dark in order to withstand a giant twister, after which Diane easily defeats Hauser, and cracks the boulder on which Byzel was located on.
Her next fight is with Meliodas. Fuelled by by Ban to lose control, Diane goes all out, and even in shrunken form which according to Meliodas should have weakened Diane, she largely overpower Meliodas and nearly destroy the giant boulder.
The Holy Knights appear and seemed to have beaten the sins. A metal robot looking Holy Knight with the power of gravity manages to throw down a deep chasm. She rises up with her weapon Gideon, and with a single punch, slams the Holy Knight flat. Seeing Meliodas beaten, she loses control. Using her Sacred Treasure, Diane displaces a mountain sized boulder from the ground about 60 times her own size and slams it on top of Helbrem, who manages to barely escape just in time.
Powers and Abilities
Diane has attributes that allows her to manipulate Earth, just like members of her clan. Usually very kind, she is quick to throw a fit and does not know how to hold back.
Strength: Diane is immensely powerful. Even without Gideon, she can destroy massive boulders, create Earthquakes and has many Earth related attributes. She can leap about half a mile without running, jumping out of a town in a single leap. One strike with Gideon ruptered the ground around sent meteor like rocks hurtling all around, destroying anything they came into contact with. More than 3 miles of Earth was destroyed by that single strike, she literally ripped the mountain off the face of Earth and had it suspended in mid air, before dropping the mass on Helbram.
Speed: Diane can easily keep up with Meliodas, one of the fastest beings in the series. And this was during a time when she was shrunk, her strength and power had been greatly reduced yet she fought on par with Meliodas.
Creation: Using this power, Diane has been seen turning the ground into quick sand. The ability is most likely one that allows her to manipulate Earth.
Weapon
Gideon: Dianes Sacred Treasure, the weapons that lets her pull out her vast reservoire of strength. It’s a massive warhammer about 10 times the size of a normal human being. Since Dianes power is creation, she can use Gideon to use the same ability with