Harrier is an ability with two effects. The first is to cause nearby enemy units to become marked as Vulnerable every few seconds. The second is to cause Quinn's first autoattack against marked units to consume the mark and deal bonus damage.
Valor is considered to be in combat upon being damaged by an enemy basic attack or ability or damaging a unit with a basic attack or ability.
While Tag Team is active, Valor's basic attacks are considered as melee for the duration.
Quinn and Valor are effectively considered to be the same unit by the game - as a result, they will share the same champion statistics, cooldowns on their basic abilities (unlike many other shapeshifter-type spells) and will both die when one is killed.