red shirts

Red shirts are expendable characters in fiction, especially minor crew members in Star Trek who die soon after appearing—they wore red uniforms.

The term now applies broadly to any character introduced only to be killed off, or to real-world situations where someone seems set up to fail: Don't send the new guys in first—they're just red shirts.

In American college sports, to redshirt is to hold a player out of competition for a year to extend their eligibility.