In order for you to receive the Soul Eater you must have an empty slot in your inventory or you wont get it simple as that.
preferebly complete all the quests so that you may choose any quest you like, however if you cannot do that yet make sure to do them in this order
1. Dr.Osto's Research (mines)
2. Unsealed Door (mines, followed by caves boss so be careful)
3. Waterfall Tears (caves)
4. Black Paper (caves)
5. From The Depths (ruins)
okay so heres what you do
1. in the quest "Dr. Osto's Research", Do Not tell Sue your name or this will not work.
2. next in the quest ''Waterfall Tears'', defeat Anna, then you must continue on and you will find Kireek further up the map. if you cannot find him look at the map and there should be a green arrow indicating him.
3. In Cave1 of the ''Black Paper'' quest, you can find Kireek near the teleporter to Cave2. He is in a dark room beat him down.
4. now, complete the quest ''From the Depths'', and make your way up to Ruins2. close to the start of the map, you will see two arrows indicating NPCs (non-playable characters). go to them for the last battle with Kireek. beat him down like he deserves it and you will get his weapon, Soul Eater, which funnily enough every character can use, good for you force guys out there i guess.