noun
selfish person
A person who acts according to their own desires without considering others; often used in a mildly critical or affectionate way.
彼はわがまま者で、いつも自分の意見を押し通す。
He is a selfish person and always insists on his own opinion.
あの子は少しわがまま者だけど、憎めない。
That child is a bit selfish, but you can't hate them.
わがまま is the adjective/noun meaning 'selfishness' or 'selfish', while わがまま者 specifically refers to a selfish person.
Compound of わがまま (selfish) and 者 (person).