Skaven characters can choose to lead units from the rearmost rank, considered a place of honour by Skaven (remember that Champions are not characters). The unit can use their Leadership as normal. Battle Standards have the same effect that they would have if they were in the first rank. Characters can issue and accept challenges only if in base contact with the enemy, as normal.

If their unit is engaged in combat from a previous turn and the Skaven characters are not in base contact with the enemy, then they have three options in their next Movement phase. They can either stay at the rear of the unit, move to the fighting rank, or decide to leave the unit and move out of it (as if the unit was not engaged in combat). This doesn't mean that they can join units engaged in combat.

Furthermore, if Skaven characters refuse a challenge, the Skaven player, rather than his opponent, can choose which character goes at the back of the unit. Once this character is moved to the back of the unit, no further penalties apply to him for having refused the challenge. Note that if the rear of the unit is engaged in combat, the character has to be placed on the flank, inside the unit, or in any other position where it cannot fight. You can never get into base contact with enemies when refusing a challenge.