I noticed that there have been some comics where Azu-Nyan was just randomly there...but they never actually explained when they met her yet...so maybe there are some missing pages? :/
Maybe Azu-nyan was introduced in new strips in the printed volume
...'stay warm'...You shouldn't be out of bed!Yui-chan, I'm sorry for worrying you...Ui, don't overdo itWaaah~!!W-Welcome everyone. Alright, here is lunch..S-sorry, it's just crisps today...MugiYuiRitsuUiUi-chan caught a cold~!!If I just stay warm and get plenty of rest, I should get better in no time.Everyone... I'm glad...I'm so happy, I'm feeling fever might be going down...