The main character should always be in the foreground, it is an integral part of the story. However, you should not mention the name of the hero in the logline, unless it is known in the whole world — for example, John Lennon or John Kennedy. Otherwise, the name won't tell the reader anything. It is better to indicate his profession, social status — homeless, billionaire, architect, etc. In the case of "The Karate Kid" - this is a teenager from New Jersey.
Use well-chosen adjectives that can characterize the hero of the story in a better way. For example, conceited, ambitious official, meticulous journalist.
The main character in the film can be not one person, but a group of people. In such a case in a logline they may appear as a single hero in the logline.