Her unfinished business is probably waking her parents up for yet another glass of water before bedtime
A wraith is the ghost of someone who has died, but whose soul felt it still had unfinished business in the realm of the living, and thus who has been unable to leave reality behind completely and enter one of the many available forms of afterlife.
Each wraith's continued existence on the mortal plane is tied to a Fetter, an object or person important to them in life, and which is somehow symbolically tied to whatever unfinished business they feel they need to attend to. Once their Fetter is destroyed, or their unfinished business is brought to a satisfactory completion, a Wraith dissipates, either into Transcendence (one of the good afterlifes) or into Oblivion (losing their grip on reality but not their presence, becoming a mindless monster known as a Spectre), depending on how the situation resolved.
A wraith's body is not made of matter, but of Plasma, a magical substance which is not normally solid, but which can take on almost any properties or appearance. Slice a wraith in half (which is quite possible) and you would find, not internal organs, but undifferentiated "stuff" all the way through their body, no matter how much it resembles their living form on the surface. Wraiths can be visibile or invisible as they please, pass through most objects, and are not affected by gravity unless they wish it, but can momentarily become solid enough to hit an opponent in melee combat, pick up objects, or speak to the living. But, of course, while they are solid enough to affect normal matter, they are solid enough to be affected by normal matter, and can be shot or stabbed or pushed around. However, they take very little damage from such attacks, unless the annoyance and inconvenience of such attacks prevents or delays them from doing something they care about, because such frustration harms their mind and soul far worse than any mere physical damage.
Additionally, each Wraith has a sort of split personality. The Psyche, or ego, is most closely linked to their personality they had before they died, and is generally the part in charge. The Shadow, or id, is the focus of their negative emotions, self-doubt, and self-destructive tendencies, and actively seeks to hold them back from completing their unfinished business, and drag them down to Oblivion. The Eidolon, or superego, is the reflection of their higher self, constantly working to get them to abandon their ties to life and peacefully achieve transcendence and move on to the next plane of existence. Generally, the Psyche and Eidolon work together to keep the Shadow in check and fulfill their objective so they can then transcend, but if they are thwarted in their goals the Shadow can become too powerful to hold in check.
Despite their ties to Reality, Wraiths spend the vast majority of their time within the Umbra, wandering the labrynths of the lower levels of the Deep Umbra, popping out occasionally into Reality to haunt the living and seek closure with whatever unresolved issues are keeping them from eternal rest. Some Vampires, especially those of Clan Giovanni, practice magical arts of Necromancy and attempt to bind and control Wraiths, often by means of seizing control of their Fetters. They either promise to help the Wraith resolve their unfinished business, or threaten to destroy the Fetter and prevent the Wraith from meeting its goals, in order to cajole or blackmail the Wraith into doing the bidding of its master. As wraiths can be invisible, pass through walls, and momentarily become solid enough to steal objects or attack otherwise unreachable targets, wraiths can make quite useful servants for those who can truly control them. Other Vampires, and especially certain Mages, have mastered the art of Soulforging, and can actually melt Wraiths and forge them like steel, creating tools, weapons, and other objects with immense magical power. This process of course destroys the mind of the wraith involved, and is akin to a death sentence for the already technically dead.
Behind the Scenes
Wraiths appear rarely in the Tech Infantry universe, because quite frankly we never bothered to buy the sourcebooks for Wraith: The Oblivion, the World of Darkness game focusing on such creatures. They are not generally available as a player character race, but do occasionally show up, usually as the minions of some powerful Vampire antagonist. The only named wraith currently active in the game is Hugh Montgomery.