When defining a game system, I've found it useful to organize my thoughts around this structure. Each of these categories should enhance fun for players and provide storytelling opportunities.
The above chart, from FM 22-100, illustrates the US Army's definition of a heroic leader. We can expand these definitions to apply to all characters, regardless of their heroic or anti-heroic inclinations.
According to FM 22-100, this includes internal qualities possessed at all times, whether alone or with others. At the basic level, that means two things: values, and attributes.
Whatever attributes a system includes - Strength, Dexterity, Soul, Logic, Luck, Hit Points - it doesn't matter, they all fall in here. Other parts of a character's intrinsic being can be included here - many FUDGE Gifts, for instance, would qualify as attributes regardless of whether they are rateable. A character's choice of race, and the associated traits of that race, also fall into this category.
Values include alignment or devotions; they answer the question of why. Why does the character adventure? What does he fight for? What is she willing to die for? It doesn't matter if a system has mechanical rules for these things, they are a part of every character.
In a word, these are skills. Not every system has skills - but they probably each have something that falls into this category. D&D 3e/4e, Pathfinder, RuneQuest, and FUDGE all have skills. Phoenix uses specialties. D&D 2e has non-weapon proficiencies or secondary skills.
In D&D 0e/1e or clones like OSRIC, there is not much specific mention of skills. Rest assured, they are there. A character's ability to climb a cliff face might not be called out as a specific skill in every system, but that usually represents a simplifying assumption that such a task is always roughly equal to that character's Strength score.
Some class abilities might be a part of Know.
This is the application of knowledge - or in most games, a character's class. Other terms might be profession, job, position, template, or OCC. Of course there are classless systems, but even in these, there is some expression of how a character applies their knowledge - especially in combat encounters. Spells. Attacks. Backstabs. (preferably with ballistas)
For those who aren't making a system from scratch, but rather using one already in existence, I believe keeping these categories in mind helps create rounded characters. NPCs can especially use this - a quick thought on "Be, Know, Do" for an NPC gives the GM a framework that doesn't need much else for most roleplaying scenarios.
- Rok is strong, motivated by donuts, skilled at fishing, and works as a town guard.
- Mojia is lucky, motivated by forgotten knowledge, skilled at calligraphy, and works as a shipbuilder.
These two characters are instantly unique and playable - the rest can be improvised by most GMs.