have a case on (someone)
To have a romantic infatuation with someone, especially unbeknownst to that person. I've had a case on Tommy for years.
have a case on
Also, have a crush on. Be infatuated with someone, as in He's had a case on her for years, or Teenage girls often have a crush on this teacher. The first slangy term dates from the mid-1800s; the second, a colloquialism, dates from the late 1800s.