Soukoku is a duo put together by the Port Mafia boss Mori Ougai that consists of Osamu Dazai, an intelligent 16 year old boy who is has the special ability no longer human to nullify other special abilities and Chuuya Nakahara whose ability for the tainted sorrow=gravity control.
Even though there's rumours about them dating they are only seen bickering. 
Dazai is the brains while Chuuya is the Brawns
Chuuya can unleash Arahabaki but loses control over himself, only Dazai can bring him back.