On the contrary: 反而
副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
反而 (fǎn'ér) = "on the contrary, instead". The result is the opposite of what you'd logically expect.
It goes after the subject, before the verb. Often preceded by 不但不 / 不但没: 吃了药，病不但没好，反而更严重了 (after the medicine, not only did it not improve, it got worse).
Nuance with 却: 却 marks simple contrast; 反而 says it's the very opposite of what was expected.
Examples
我帮他说话，他反而生我的气。
Wǒ bāng tā shuōhuà, tā fǎnér shēng wǒ de qì.I stood up for him, and instead he got angry with me.
雨不但没停，反而越下越大了。
Yǔ búdàn méi tíng, fǎnér yuè xià yuè dà le.Far from stopping, the rain got heavier and heavier.
手机让交流更方便，人们反而见面少了。
Shǒujī ràng jiāoliú gèng fāngbiàn, rénmen fǎnér jiànmiàn shǎo le.Phones make communication easier, yet people actually meet less.
Common mistakes
✗ 反而他没来。 → after the subject. ✓ 他反而没来。
✗ 他很努力，反而考得很好。 → the result is expected, not opposite: no 反而. ✓ 他很努力，所以考得很好。