Leaving a Unit
A character might stay with a unit for an entire battle, or might need to move on -- to take charge elsewhere, or because their current unit has been cut to pieces around them.
A character can charge out of a unit by declaring a charge in the Charge sub-phase -- the character moves and the unit stays put, unable to declare a charge of its own that turn, though it can still move normally in the Remaining Moves sub-phase. Only one character per unit may charge out this way each turn. A Stand & Shoot reaction against a charging character targets them as a separate target, with none of the protection of being in a unit, though they still get a 4+ "Look Out, Sir!" if leaving a unit that shares their own troop type.
Alternatively, a character may simply leave their unit during the Remaining Moves sub-phase, moving away before the unit makes its own move -- impossible if the unit can't move that phase for any reason, and never on the same turn the character joined. A character can't leave, for example, if their unit is fleeing, if a spell or effect stops the character or unit from moving, if the unit is in combat, or if it's made a failed charge.
Once clear of the unit, the character reverts to their own normal movement rules, measured from their actual position before the unit itself moves -- they may even join a different unit as part of the same move, movement allowing, though that new unit can't move that phase unless it already has.