You can do this only once per difficulty setting. To get Kireeks' Soul Eater, you need to play "Dr. Osto's Research" quest. Then, after you get the information at the end, talk to Sue. If she asks your name, say "No". After you finish that quest, go to the "Unsealed Door" quest. You will meet Sue again. Talk to her and listen to what she says. Then, complete the quest. Next, go to the Waterfall Tears. After defeating Anna, continue on to Cave 2 (same quest) and you will eventually meet Kireek. He will greet you, then you will have to fight him. After you defeat him, go to the "Black Paper" quest where he will try a rematch in Cave 1 of this quest. Note: You are in the dark in this battle. When you defeat Kireek in that battle, you will meet him again in the "From the Depths" quest. In that quest after you separate from Ash, go back to Ruins 2 (in this quest) and go as far west you can. You will confront Kireek for the final time. It is recommended that you equip some armor, shields, units that can withstand Dark/Draw/Shadow or whatever that weapon is made out of, or use Razonde (or Zonde, if that is all you have). When you defeat him you get to keep his weapon for your accomplishment.