Ghosts have the ability to affect the world around them.
Tapping the touch screen will have various effects on enemies and players. For example, tapping (and then holding down) on an enemy will lower its defence, while tapping and holding down on a player will increase their attack. When the dead player touches an enemy or ally, a faint Wraith Skull appears at the location tapped. This means that players still alive can tell what the Wraith is focusing on. Note that the screen must be held for the effect to be continuous. Players can affect friendly players, Archfiends, and regular monsters too.
Buffing and debuffing uses a a portion of a replenishing bar, located at the top of the screen. If the bar is empty, no buffs or debuffs can be applied to allies or enemies until the bar regenerates.
In addition to this, dead players can also view damage to enemies and allies in numerical form, and see the health bar of Archfiends, giving them accurate information on the state of the quest. In this fashion, defeated players can still contribute to the success of a mission.
When becoming a ghost, there are some points to consider when trying to help your teammates, namely how many allies are left, how many archfiends your team is fighting, and what your allies are actually doing. There's no point buffing the attack of an ally who's casting a healing spell, nor is there a point in debuffing a monster that isn't even in combat.
One ally fighting one Archfiend - Either boost the ally's attack while they're attacking, or drop the Archfiend's defense while the ally is attacking. Pay attention to the ally and stop buffing/debuffing when they're not attacking, to let your ghost bar regenerate.
Multiple allies fighting one Archfiend - The main focus here is on debuffing the Archfiend, that way every attack by every ally will have more impact. The Archfiend is probably going to be attacked constantly, so it's a good idea to keep half to a third of the ghost bar always full, in case an ally uses a particularly powerful attack that you can take advantage of.
One ally fighting multiple Archfiends - Focus on buffing your ally. When facing mutliple Archfiends at the same time, your ally is likely to take moments to run away and regroup, heal, restock on offerings etc. which will give you time to regenerate the ghost bar.
Multiple allies fighting multiple Archfiends - This is the trickiest scenario, as there's no set way to act when you're a ghost. Try to watch the numerical damage being done to the Archfiends, and focus on debuffing the Archfiend that's already the weakest. For example, it's a good idea to help debuff the weaker Archfiend (weaker by design, or by virtue of elemental vulnerability), to kill it faster and leave your teammates more able to take down the stronger Archfiend.
NPC characters that the player sacrifices will not use any Ghost mode abilities. The NPC will simply stand at the spot they were sacrificed at.