If there is one thing that makes the Archangels a very unique bunch, it would definitely be their quirkiness. Each one of them was different from normal people in one way or another. That's why people feared them and saw them as a threat even at the level of Virtues.
They were a borderline insane and psychotic bunch who could do unexpected things at unexpected times. However, in this group of misfits, Arthur stands out like a sore thumb.
His appearance, demeanor, character, and everything about him looked so… underwhelming compared to the others. He was by far the most sane-looking Archangel, however, considering how the others were simply crazy, the fact that he was the only one who didn't look the role was unnerving.
As far as the others knew, Arthur was an average kid from an average family who only discovered his abilities when he was around 12, and then because of Divinity's offer to his family -which was money-based- he joined as an ascender.