what does に do in いつの間にこんな自立した子に…


I don't understand why に is used in this sentence I saw this sentence in the
second episode of the anime k-on

> いつの間にこんな自立した子 **に**...うれしいような悲しいような

the English subs said that it meant

> She's all independent already.It makes me both happy and sad




As others pointed out, this is a case of omission. In Japanese, if the intent
of the statement is free of ambiguity, you can cut out the rest.

> いつの間に【この子は】こんな【に】自立した子に【なったのだろう 】。【私は】うれしいような悲しいような【気持ちです】。

