It took about 5 pages for Shimamura to agree to spending Christmas with Adachi.
Adachi wants to be “special” friends with Shimamura, enough that they can spend Christmas together and walk around with their fingers intertwined.
Shimamura really could not give a rat’s ass. She only agreed to go with Adachi on Christmas because she thought Adachi had no other friends (well, she was right though) and was lonely. During Christmas, Adachi tries to hold her hand and Shimamura slaps it away then lectures her about asking her first because she didn’t know if some pervert grabbed her or something.