Signature Skills

Define Your Character
The first commercially available role-playing game, the game that started the hobby, is class based. That means each of the the player characters has a specialized role in the game, such as fighting man, cleric or magic user - just what the class name implies. In 1978, The Chaosium published RuneQuest, a skill based system using percentile dice and with no character classes. The system mechanics introduced in RuneQuest have become a popular alternative to the class based system and it its basics were boiled down into a short pamphlet titled Basic RolePlaying, which eventually became the basis for Call of Cthulhu and other popular role playing adventure games published by Chaosium.
A skill based system is wonderfully flexible and soon other games borrowed the idea and adapted the mechanics for there own unique take on role playing. Systems such as GURPS, which although abandons the percentile dice in favor of the bell-curve of rolling three d6 uses the skill based approach to character building. Players are bright folks and they quickly discover which skills give characters useful advantages and as a result, many characters are built with high skill competency in the desirable skills, making many characters very similar in that regard. Some players try to make their character equally good at all skills resulting in basic competency, but nothing exceptional.
The way to have a specialist, a character who stands out from the crowd as the go-to PC for a certain type of situation, is to build a small number of clustered skills up to an exceptional level, probably at the expense of neglecting other skills. In this way the character has a specialty, a role to play that is connected to their signature skill(s). Yes, this imitates the class system, but with a bit more flexibility because the character can potentially become excellent at any skill on the list.
Advanced skill system games such as the latest iteration of Chaosium's Basic RolePlaying (BRP) offer a number of ways to individualize a character. Advantages and disadvantages, quirks and passions, backgrounds and professions are all ways to individualize a character either during creation or advancement, but are often cosmetic and just offer color to role play, which can itself be fun. What defines your character's prime ability and principle role in the game requires being better at something than anyone else in the group of adventurers, however. This can take some work on the part of the player and frequently involves sacrifices in the form of not being very good at anything else. That's why there are other characters in the party. There is a certain satisfaction in being able to shine when it is your tun to do your thing. Being able to reliably succeed when called upon is a nice feeling. And for the rest of the time, that's when we have the fun of role-playing and being a supportive cast member.