Characters who are part of a unit must obey all the usual coherency rules, meaning that they must remain within 2" of a member of their unit.
Independent characters are allowed to move freely around the battlefield and can temporarily join other units if desired. While an independent character is part of a unit, they must obey the usual coherency rules until they leaves the unit as detailed below.
If a unit falls back with an Independent Character of a different unit type attached to it, it falls back at the speed of the slowest unit.