At the beginning of part two, the Death Devil is already on Earth and attends Asa and Denji's school, plus she has already an agreement with the Fire Devil (disguised as the Justice Devil) to make him do pacts with the students at the school
Death Devil notices the arrival of War Devil on Earth and the fact that she has taken the form/body of Asa
Shii orders the “Justice Devil” (the Fire Devil) to make a pact with Yuko, who becomes aware of Yoru's identity in Asa
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(this last image is from Chapter 106)
Through Yoku's pact with the Fire Devil, Shii is able to study Yoru's current abilities, now that she has descended on Earth