At first I thought she was unfriendly, but actually she's just shy.

at 是介词，表示"在...时候"或"处于...状态"。 first 是序数词，表示"第一"或"最初的"。 at first 是一个固定搭配，表示"最初"或"一开始"，用来描述事情最初的状态或看法。