*might contain spoilers*
Akutagawa Ryunosuke, is a member of the underworld organisation, The Port Mafia.
His ability Rashomon, which allows Akutagawa's coat, and any other garments he wears to transform into an omnivorous, shadow-like black beast.
Akutagawa is shown to be the one of the most formidable member of The Port Mafia, due to ruthless and abhorrent attitude. He is basically ferocious and barbaric when it comes to wiping out The Mafia's enemies. He has a very plain and sickly appearance and he is always coughing though he is only 20 years old. His view of the world is rather very simple or cruel that the strong should get stronger and the weak shall remain weak.
He always remains detached from social interactions, and mostly remains composed and mostly behaves with everyone alike. Though he is talented and skilled and his ability is beyond the definition of power, there is a great feeling of self doubt in him, the sense of inferiority was always there no matter what he accomplishes. This anger and self-doubt of his is risen from the fact that he was not approved by one certain person who he respected the most.
When he was 14 years old, he was picked by Osamu Dazai, from the slums of Yokohama, since then he raised him, I would not say with utmost care but with utmost flare. He was always rebuked by Dazai for not being able to fully activate his ability and was never approved by him, still Akutagawa looked up to him as Dazai was the most respectable person in his eyes due to his bloodstained reputation in the Mafia. But since Dazai left the Mafia, Akutagawa thought of his defection as a disgrace. He was also deeply wounded when he saw Dazai's high opinion of Sakunosuke Oda, who refused to kill anybody even if he was a memeber of the Mafia.
Presently Akutagawa is obsessed for killing Atsushi, because he is a Port Mafia's target and moreover he is Dazai's new apprentice who has been approved by him. This fact fills Akutagawa with jealousy and hatred towards Atsushi. When Akutagawa first meets Atsushi, he brands him as a weak one who doesn't own any place in this world and is destined to die. But his opinion for Atsushi worsens more and more, the fact that he is Dazai's current pupil makes Akutagawa go mad every single time.
Akutagawa is basically a kind of person who doesn't value someone who is weak, and he doesn't judge people by their character, and maybe that is the reason he never got approved by Dazai. He never understood why Dazai has an high opinion for people who he thinks as weak, though it seems that he is mature from his outer appearance but somewhere it's his imaturity that makes him judge people based only on strength and which is why he never understood that even after being powerful and talented why he didn't earn the approval and praise of the person who he respected the most.