The Ruins - Flowey, Toriel

Snowdin - Papyrus, Sans (boss fight ends quickly with just a warning)

Waterfall - Monster Kid and Undyne

Hotland - Alphys

New Home - No one

Judgement Hall - Sans, Papyrus (if you kill sans in the warning)

Throne Room - Asgore, Flowey, Photoshop Flowey, Asriel.