From the moment of birth, Rasiel – as the firstborn – had been the favourite. Belphegor, born minutes after his brother, had been destined from the very moment his older sibling entered the world to be the neglected twin. If positions had been reversed and Belphegor had been born first, things may have been different, but fate hadn't played those cards.
From a very young age, Rasiel had picked up on his brother's mistreatment. He could see the way their parents ignored Belphegor, refused to give him toys or anything of the sort, and even go as far as to – on the occasions they acknowledged his existence – physically abuse him and put him down.
Rasiel had, unfortunately, fallen into his parents' habits. He copied their behaviour, but instead of treating Belphegor as invisible, he got more pleasure out of hurting his twin, telling him how pathetic and useless he was – how he would never be as good as his big brother.
Belphegor always rose to the bait, and whether it was throwing stones or knives at each other, Rasiel always won. To see his brother visibly upset after the losses, the older twin had become addicted to seeing his brother's sadness – an influence of their parents' hatred towards their youngest son.
If they weren't physically attacking each other, Belphegor would sit silently in their shared bedroom, his hands in his lap as he stared downwards, not responding to Rasiel's cruel words and insults. There were the times Belphegor would twitch randomly, as if he wanted something but was fighting to restrain himself, much like a kid who wanted to touch but knew they weren't allowed.
Rasiel was nasty, and as much as his brother would deny it just a few years later once he would join the Varia, the damage had already been done.
For years, up until the twins were eight-years-old, Belphegor only ever seemed to fight back physically; he would remain silent when it came to verbal lashings, giving the impression that he was thinking very hard on the things his brother said to him.
Belphegor would never tell anyone about the things his brother said to him. Rasiel would tell him that he should kill himself because no one would miss him, that no one loved him and wanted him around. He had been called some of the cruellest names on the planet, and the truth was, Belphegor believed the things his brother said to him.
Rasiel knew this. He knew that he got under his sibling's skin, but what he would never know was that Belphegor would go through his entire life unable to love or trust anyone, constantly believing that, as he had been told, he wasn't wanted or loved – not appreciated in the least by anyone.
The older twin loved the nightmares he caused his brother to have at night. He would lay awake laughing as Belphegor screamed and cried in his sleep, whimpering for Rasiel to stop talking.
But as smart, as 'superior' to Belphegor as he was, he didn't quite comprehend the fact that he was slowly pushing his brother over the edge – not until that day where his twin just snapped and stabbed him over and over again.
For the first time in his life, Rasiel had lost to his brother, had feared the younger. He had been left for dead out on the castle grounds while his brother killed their parents and all of the servants who worked for them, only for the older twin to be rescued and taken in by a man named Byakuran.
Despite how much pain he was in as he was carried away and later given medical treatment, Rasiel could only laugh in a crazed manner at the knowledge that he had been the one to break his brother's sanity.
Belittling Belphegor was one of his pleasures, after all.
To be fair, I don't think the timeline of the series would allow for Byakuran to rescue Rasiel or not, but seeing as we don't know his status in that time period, I like to think that he was rescued by Byakuran, which is why he was one of the fake Funeral Wreaths in the Future arc.
I've always viewed Bel and Rasiel's relationship as one influenced by neglectful parents and a favoured twin. In fact, I've pondered the idea that Bel's instability was caused – or aggravated – by his childhood, and that Rasiel's own hatred and superior insanity was a result of his parents' neglect to his brother and later fuelled by Bel trying to kill him. Has anyone else considered this?