〜くせに
even though (and I don't like it)
Plain form + くせに (な-adj + な · Noun + の)
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くせに is like のに, but with open criticism of the person: 知っているくせに、教えてくれない, "they know perfectly well, yet they won't tell me".
It's aimed at people, and it says their behaviour doesn't fit what they are or know: 子どものくせに, "for a mere kid"; お金がないくせに, "when they have no money". The subject of both halves must be the same person.
Because it's so judgemental, it's mostly used when complaining about someone, or in arguments. On its own at the end of a sentence, it's a sulky retort: 知ってるくせに!, "you know perfectly well!"
For a neutral "considering", use わりに; for mild disappointment, のに.
Register: Critical, and rude if aimed at someone directly.
Examples
知っているくせに、教えてくれない。
They know perfectly well, yet they won't tell me.
子どものくせに、生意気だ。
You're awfully cheeky for a kid.
自分ではやらないくせに、文句ばかり言う。
He never does it himself, yet all he does is complain.
下手なくせに、歌いたがる。
They're bad at it, yet they always want to sing.
お金がないくせに、高い物ばかり買う。
He has no money, yet he keeps buying expensive things.