so my theory is that, since Yousuke never actually got taken over, the two souls were both dwelling inside a single body in a sort of supernatural balance and at the end there the real Jormungandr took in the poison and left as much of his own power as Yousuke could withstand behind before being consumed and disappearing, sort of a "look after my sister for me" gesture, and as a result MC got active control of that power but kept his own personality
and as for why Jormungandr would do such a thing, perhaps he recognized the MC's will for peace and acceptance of all supernatural beings, but due to his own nature couldn't carry out such a thing, and thus chose to entrust MC with power in the hopes of that vision coming to be