Born in 2079, just two years after the Great War, Lily lived a simple life in Vault 17- until the Master's army raided the vault. The 75 year old grandmother was dragged outside, where she saw the sun for the first time.
Lily now tends to the bighorners in the mutant community of Jacobstown. Like all nightkin, she struggles with her dangerous schizophrenia brought on by excessive Stealth Boy usage, but is aided by Doctor Henry, who considers her to be a primary subject for medical testing in his personal quest to cure the nightkin of their affliction. Unbeknownst to Henry, Lily only takes her medicine in half-doses as it allows her to remember her grandchildren, a recording of whom she carries around on an old holotape.
Lily frequently mentions someone named Leo, who seems to be the manifestation of her schizophrenia. She will have conversations and even arguments with "Leo," insinuating that he drives her to do "bad things." Leo's voice is audible to Lily only, but she experiences it as real; "How else could I hear him tell me who to chop?" In contrast to Lily's generally helpful and friendly personality, whenever she begins to exhibit violent tendencies, she will blame Leo.
As it was mentioned by Doctor Henry, Lily is mad but not dangerous. Leo, however, is all about aggressive and sociopathic behavior.
Leo is very fond of melee weapons. He is the one who tells Lily "who to chop," and he will not be satisfied if Lily doesn't engage in melee combat on every opportunity she gets. If ordered to favor ranged combat over melee or to take a passive stance rather than an aggressive one, Lily's reply will be, "Well, if you insist. But Leo is not going to like it much!"
Lily also tells the Courier that Leo was the one who instructed her on making her Vertibird sword using parts scavenged from the Vertibird wreckage in Klamath back in west California. This weapon was built from the wreckage of a Vertibird previously flown and subsequently crashed by pilot Daisy Whitman.
It is mentioned by Marcus that the nightkin suffer from schizophrenia induced by their prolonged use of Stealth Boys. During dialogue with Lily, the Courier may ask about the location of Leo. Lily replies by saying that he's right there by her side. The Courier may then try to convince Lily that there is no one there (to no avail, of course).
The creation of imaginary friends seems to be common among the nightkin, Davison takes his orders from a dead brahmin skull which he calls Antler, the unnamed nightkin that guns down Novac's brahmin every night, arguing with an unseen being, and Tabitha's radio show is nothing but Tabitha talking to herself as she alternates between her own voice and her imitation of the voice of Rhonda.
Lily has to regularly take her medicine in order to "make Leo shut up," and prevent him from taking over control of her body and going on a rampage in the process. If the Courier chooses to make Lily stop taking her medication, she will start to become more effective in combat but also more prone to psychotic rages, and, in the end, she will be little more than a ravening beast. Lily and Leo's condition is similar to the one of Dog and God.
Lily and Leo: At a certain point, if Lily Bowen is the active companion, the player may begin to hear a recording of children greeting their grandmother. This signals the start of Lily's unmarked quest.
Guess Who I Saw Today: Lily is asked to come into the Jacobstown Lodge so Dr. Henry can do a couple of experiments on her, concerning the effects of Stealth Boys.
Encouraged by the Courier to take her medication regularly, Lily's mind eventually attained a semblance of clarity. Her memories dulled by the pills, she cast aside the recording of her grandchildren, no longer remembering its significance.
Encourage Lily to take her medication regularly in her unmarked quest, Lily and Leo.
Lily continued to take her medicine at half-doses, and although she remembered her grandchildren, her mind remained muddled and confused. Eventually, she parted ways with the Courier and traveled west, seeking the remnants of her past.
Encourage Lily to continue taking her medicine at half-doses in her unmarked quest, Lily and Leo; or do not complete the quest.
Encouraged by the Courier to stop taking her medication altogether, Lily's instabilities grew worse with each passing day. Eventually she became little more than a ravening beast, the kindly old grandmother subsumed beneath the rage.
Additionally, Lily has a third companion weapon, a power fist variant called Lily's gauntlet. This weapon is not initially in Lily's inventory, but may be given to her as part of a script which ensures all companions retain their special weapons.
Lily is limited in the weapons she can use, and can only theoretically equip weapons from the following list. Note: Lily's default assault carbine does so much base DPS that she will never equip certain weapons in practice; these weapons are crossed out.
Due to the above limitations, there are only two holdout weapons which Lily is capable of using: frag grenades and plasma grenades. This makes Lily a less-than-ideal companion when performing quests which take place in "weapon-free" areas like the Strip's casinos.
If you enter with Lily inside the cafeteria of Vault 3, she will comment about herself when she was a small girl and she will remember her first kiss. This will also occur upon entering Vault 11, Vault 19, and Vault 22.
As a nightkin, Lily is also significantly faster than the player. It is not uncommon for her to charge ahead to engage a target, which can be problematic when dealing with creatures like deathclaws.
When Lily is a companion, if she is reduced to one half of her hit points, she will suffer a "psychotic break" and will be in aggressive mode until it fades. The player is unable to give Lily commands or interact with her until this break subsides.
While Lily is not a companion, cazadores may attack Jacobstown and kill her.
If the player asks her to be aggressive she will comment saying, "That's eactly what Leo told me to do!" The word 'exactly' is misspelled.
When you go to the Tribal Village with Lily, she will ask "Oh Dear, did I do all of this?"
When enemies are identified, Lily will frequently state "Get behind Grandma" and then place herself between the player and the enemies. This can prove frustrating when trying to use a ranged weapon to dispatch enemies.
When crouched, Lily enters her own sneak mode, where she activates a permanent stealth boy, until the player stops sneaking. When in this mode, it is hard to see Lily in front of you, or between you and an opponent, and the player can frequently shoot her accidentally, because of her tendency to stand between the courier and his/her enemies. This can sometimes be countered by telling her to "back up" in regards to the player.
The following is based on developer test content cut from Fallout: New Vegas and has not been confirmed by canon sources.
pcps3xbox360 Lily's Vertibird sword and assault carbine may occasionally clip into one another, causing whichever one she's using to have the other hanging from it. The sword may also be replaced entirely by the gun. She will not shoot with the gun, however; she will continue to use it as a melee weapon. Opening her companion wheel and forcing her to switch between melee and ranged attacks a couple times will fix this glitch. [verified]
pcps3xbox360 Sometimes when going into stealth mode, Lily will say her normal line, "Oh, don't you worry about me dearie, your grandma knows how to stay quiet," but will repeat it every 20–30 seconds. [verified]
xbox360 Even before recruiting, if you talk to Lily, she will comment on things like Lucky 38 or Benny being killed. [verified]
xbox360 After killing the mercenaries that are at Jacobstown she may randomly attack the player then fall unconscious. [verified]
pcxbox360ps3 On rare occasions, Lily may die before she is recruited as a companion in a non-hardcore game. [verified]
pc Sometimes, when Lily is equipped with Oh, Baby! she will randomly lose the weapon. It will no longer be in her hands or in her inventory, where it would normally go if it were to break from use. [verified]
pc If Lily is asked to use ranged weapons, she may not take out her assault carbine, and instead stay with her vertibird sword, or bumper sword if it is in her inventory. Even if switched back to melee, she will still stand near the courier, trying to use a melee weapon as a ranged weapon. This can be fixed by switching her back to ranged, then back to melee again; however, there is not a fix to make her use her Assault Carbine, because the next time you switch her to ranged, the bug occurs again. [verified]
xbox360 If the player dismisses Lily to the Lucky 38 penthouse, she wil not always make it there, but instead found in the Casino section, able to recruit again and dismiss to the penthouse without issues. [verified]
pcxbox360 When the player character is sneaking, Lily may not enter sneak mode along with the player, and instead will walk along as if the player was not sneaking. [verified]
pcxbox360 If the player dismisses Lily while he/she is sneaking, Lily will remain in stealth mode until recruited again.[verified]
pcxbox360 Lily may disappear on the way to Jacobstown during Guess Who I Saw Today, if she is sent to the Lucky 38 and then asked to take Doctor Henry's test for the Stealth Boy Mark II without re-hiring her. However, it will still be possible to complete the mission regardless of whether or not she's actually there, though she may not be hirable again unless somehow found. [verified]
To fix this, either access the dismissal terminal and dismiss all companions, or reload a previous save. The former should bring Lily back to the Lucky 38's presidential suite.