If a character is part of a unit that includes at least five rank-and-file models (ie, non-characters), he cannot be shot at. Any shots against the unit will hit ordinary troopers and not the character. If the unit contains less than five rank-and-file models when a ranged attack is targeted at it, then hits are allocated before rolling to wound. First, divide the number of hits evenly between all the members of the unit (including the characters). If after this there are still some hits left over, or if the number of hits suffered by the unit is smaller than the number of models in it, randomise which models are hit with a dice roll.
For example, if a unit composed of three rank-and-file models and two characters suffered seven bits, then each model would suffer a single bit, and you would allocate the remaining two hits randomly between the five models by rolling a dice for each bit (re-rolling any result of 6).
If a character model (including his mount) has a unit strength of 5 or more, then he can be picked out as a target regardless of the rules just given. Enemies may freely choose to either shoot at the character or the unit he has joined.