Missile attacks (except templates) cannot hit a character in a combined unit if there are five or more rank and file models from the same troop type (excluding other characters) left in the unit. We assume that the enemy cannot pick the character out. All hits are allocated onto the unit's rank and file models.
However, if the character has a different troop type with a higher Unit Strength than the rest of the unit, it possible to target that model separately from the rest of the unit. If so, roll a D6 for each successful Hit on the character; on a 4+, that Hit is allocated to the unit it is with instead. This applies even if the unit can only draw Line of Sight to the character, but not the unit they are with.
If there are fewer than five rank-and-file models left in the unit there is a chance that any characters in the unit could be hit – the controlling player decides who is hit, but must allocate one hit on each model before they can add a second hit on a model; they must allocate two hits on each model before they can allocate a third, and so on.