conjunction, particle
Conjunction/particle expressing criticism, surprise, or dissatisfaction that something is contrary to expectation. Often carries a negative or accusatory tone. Attaches to the plain form of verbs, adjectives, and nouns (with な/の).
彼は何も知らないくせに、偉そうに話す。
He talks arrogantly even though he knows nothing.
あの子は小学生のくせに、大人みたいな口をきく。
That kid talks like an adult despite being an elementary schooler.
ながら means 'while' or 'although' but is used for simultaneous actions or mild contrast, without the negative judgment of くせに.
Derived from the noun 癖 (くせ, 'habit') + に, originally meaning 'in the habit of (doing something)'. Over time it grammaticalized into a concessive conjunction with a critical nuance.