He appears to have a similar role to the raven in Snow White and the Seven Dwarfs. However, he plays a larger role in the storyline and is in almost every scene that Maleficent appears in. In addition, unlike the raven in Snow White, he is completely and genuinely loyal to Maleficent and, unlike the rest of her servants, he is competent and easily succeeds in his task to find Aurora. He is also the only one Maleficent genuinely cares about, and is horrified when he is turned to stone. In return, he apparently cares a great deal for Maleficent, specifically seen when he fell into despair alongside her after her goons have been revealed to have been searching for a baby for sixteen years, as opposed to a teenager, which Aurora would have been at that point in time.
Unlike her goons, who Maleficent constantly abuses and treats as pathetic wastes, he serves as her confidante and assistant, more so than her lackey, joining her more often than not from scene to scene, and having complete knowledge of her devious plots. He's also second-in-command, having control over the goons, along with his mistress.
His backstory is also explored in the film Maleficent. According to his actor in it, Sam Riley, he was rescued by Maleficent after he found himself nearing death at the hands of a farmer. Out of gratitude, he offered to join her side as her minion, which she accepts. From that point on, he served as her stubborn and snappy, yet loyal, assistant.
In Sleeping Beauty, Diablo is a black raven with pink bags under his eyes. He has a sharp, long yellow beak and yellow claws.
In Maleficent, Diablo's name changes to Diaval. Diaval is first seen with the remaining tufts of black feathers attached to his body. He is later given a cloak that exposes a large claw-shaped scar on his chest. He has two arch-like scars just above his eyebrows. His hair is black in appearance, his hair part is a "V" shape from the front. When Maleficent transforms him into said animals, he always has some raven resembling features; such as a sharper nose or claws. As a bird, Diaval is entirely black.
Diablo is first seen with Maleficent, crashing Princess Aurora's christening. There, Maleficent casts a spell on her that schedules her death to come at her sixteenth birthday when she pricks her finger on the spindle of a spinning wheel. After the curse was set, Diablo and Maleficent departed. Oblivious to the villains, her rivals Flora, Fauna and Merryweather changed the curse so that Aurora would only fall into a deep sleep when she pricks her finger. Meanwhile, back at the Forbidden Mountains, Maleficent sends her goons on a hunt for the princess but fail to find her after sixteen years. It is revealed that they were foolishly looking for a baby for the entire hunt. Maleficent explodes in rage and sends Diablo to find her. He begins his search in the forest where he spots magic from wands being blasted out of a chimney (and in the process gets hit with the sparkles when he takes a closer inspection). It is from Flora and Merryweather who have been hiding Aurora in their forest cottage to keep safe from Maleficent.
Diablo, knowing about where they live, quickly flies off and sends Maleficent the news to her great delight, and the villains capture Aurora's husband-to-be, Prince Phillip, instead. They hold him hostage in Maleficent's castle but Flora, Fauna, and Merryweather break him out. Diablo quickly rallies the goons to attack. However, they stand no chance against the fairies' magic, so Diablo heads for Maleficent's quarters. Merryweather gets mad at him and turns him into stone. Maleficent emerges, planning to silence the goons after hearing all the commotion, but she instead finds his stone body, gasping in shock before witnessing Phillip and the fairies making their escape, prompting her to take matters into her own hands.
Diablo is said to remain petrified in Maleficent's castle in the Isle of the Lost. After Carlos De Vil accidentally makes a tiny hole in the magical barrier of the Isle, Diablo's curse is broken, and feeling that the Eye of the Dragon (Maleficent's staff) is awake, he travels to the city in the Isle of the Lost, looking for her mistress. Diablo informs Maleficent about the Eye of the Dragon and she sends her daughter, Mal, to recover the Eye, but since she really doesn't believe her daughter can complete the mission, Maleficent orders Diablo (without Mal's knowing) to follow her daughter and retrieve the Eye. Mal and her friends ended successfully finding the Eye, but since the Eye was cursed to make the first person who touched the Eye fall into a 1000 years sleep, Diablo takes the Eye out of Mal's hand and take it to Maleficent.