From what I've seen a lot of when it comes to Giratina as a Pokemon, is that he's quite misunderstood and not really that bad of a Pokemon, who may even be a little remorseful in a way, especially when you encounter him a second time in the Legends: Arceus game, when you catch him...
But as much as I do like that, unfortunately, that ain't the kind of guy that Gial (Pronounced like 'Gee-All' ) is.
Gial is the kind of ruthless brute who would happily rip the soul from your body and use it to wipe his own ass. Even before Aincrar (Arceus) and his daughters Elora (Dialga) and Selussa (Palkia) damned him to the Netherworlds, he was always kind of a jerk.
In fact, he didn't always used to look the way he does now either.
Very rare tales of old often described his previous appearance as 'shining white and laced with gold'. It's thought that he looked much more similar to Reshiram in the beginning.
But his hubris led to his downfall the day he betrayed Aincrar and his sisters. Having crafted the dreaded Red Chain from parts of the Guardian Lake Dieties, Gial tricked Elora and Selussa and used the chain to bound and subdue his sisters while he went for Aincrar.
He believed he could steal Aincrar's power for himself, and ended up gravely wounding the Almighty Father, ripping open his throat and nearly detatching his head from his body. Aincrar's golden, shimmering blood spilled from his wound, and Gial greedily lapped it up, absorbing some of the Almighty Father's power.
Once they found out what Gial had actually done with the parts they'd given him, it was the Lake Dieties that rushed in to set Elora and Selussa free from their binds, and all five of them joined together to fight Gial.
Though short, it surely was a difficult fight, as Gial's strength had almost doubled thanks to him stealing a little bit of Aincrar's power. But that's the key word; A little.
The boost in Gial's power started to fade quickly as he used it up, and despite his own injury, it was at this moment that Aincrar used the last dwindling amount of energy he had to aid his other children in their fight against his own twisted son.
Once he was defeated, Selussa and Elora used Spacial Rend and Roar of Time to tear open a portal to an unknown and bizarre world, as Aincrar used Judgement to send Gial hurdling into this empty world where he was sealed.
With so much of his energy and lifeforce spent, Aincrar falls into a deep slumber and disappears. Even to this day, it's unknown to everyone, other Legendaries included, where the Almighty Father has gone to or if he's truly even still alive at all.
Having spent so much time alone with only his own bitter hatred to keep him company in the Distortion World, Gial's once magnificent body became twisted and malformed, mirroring the malice in his heart.
Quick illustration I made to go along with the short story bit I wrote for the above piece.
This also showcases a glimpse of what Gial (Giratina) used to look like before his betrayal and ultimately, his damnation to the Distortion World.
A fluffy boi! One of these days, I'll make a lil' ref of his original form. ^^
Aincrar (Arceus) weeps not only for the pain of his grave injury, but mostly from heartbreak as the very thought of one of his own children becoming so greedy and obsessed with power to the point of being willing to kill him was something that never once crossed his mind.
Despite his immense power, he was in so much shock during this moment that he could hardly fight back, nearly allowing Gial to completely detatch his head. Luckily, it was also at this moment that the Lake Guardians swoop in to serve as a distraction and release Elora (Dialga) and Selussa (Palkia) from the Red Chain.