Unmeiteki Lovemeter

Kai Shizuki is an ex-idol who is considered a prince at school, but he has a secret. He has the ability to see a balloon above a person's head and understand their emotions from that form. One day, after being chased by a fan, Shizuki is unexpectedly saved by Shinozuka, who has a bad reputation at school. Even more surprising, Shinozuka's balloons are bigger and glossier than anyone Shizuki has ever seen before.
(Source: Kaiohsha, translated)
Note: Includes one extra chapter.