The rules I've been working on for years had a system where an Individual character within a unit (as a member of rank and file) could participate as per a "normal" character. The unit & formation their unit were in determined any additional positional modifiers. This would simply be an addition (or subtraction) to their OB/DB. When melee battle was joined then the individual characters would be "matched off" against opponents... which also had bonuses/penalties due to the formation their unit was in.
The size of a unit (and the size of it's constituent members/weapon) and formation determined the total number of attacks it could make against an opposing unit. This was important because it also determined the number of opponents that an individual character was likely to face based on the density of the opposing unit.
The "quality" of the actual opponents a character would face was variable based on the average "character" level of the opposing unit. The higher the "Unit level" the less variable the "character level" of the opponent. What this meant was that low level units had the possibility of containing higher level opponents, but with a lower possibility of the character "accidently" meeting them. (Chances are these were enemy NCO's rather than rank and file). ... and high level units tended to have less variable but higher level opponents.
Within a battle, I'd rated the position of the character as a voluntary choice, though in practice it wouldn't be. Normally better, stronger opponents would be "front-line"... therefore within a unit of allied lower-level the characters would be placed at the front and vice-versa with allied higher level units. Regardless, each character had the opportunity to affect the combat, but those in the middle (or rear) had less of a chance to fully use their combat abilities (or get hurt).
With reference to the effect that individual characters might have, this would go alongside the results of the unit, with their (and their direct opponents being excluded)... and then combined again when determining morale checks and whether the unit had been pushed by, or holding or was pushing an opposing unit.
