Stalker with a Crush... only this guy happens to be a little less than human. The Mailer Daemon can be a cultist, a vampire, a demon or some other supernatural critter intent on eating, ritually sacrificing, brainwashing, or "marrying" the hapless heroine.
Over the course of an episode, he'll woo the heroine, getting her absorbed in online chatting and games, and convincing her that this godsend of true love, who curiously refuses to send his picture, is her Love at First Sight... well, read. She'll trust him completely with all sorts of secrets, personal and dangerous. Her friends might try to urge her to be cautious, but she'll think "You're Just Jealous" and grow distant from them; this is usually a thought the Daemon plants and fosters to alienate her and make her easier to seduce and lure. He'll eventually persuade her to come meet him in the Haunted Castle or Mad Scientist Laboratory, only to reveal himself in his true form.
Cue The Scream.
All things considered, it's awfully insensitive and hypocritical of her to be so judgmental of others' appearances. The Mailer Daemon does this because he could never interact socially with the heroine without the Internet (he might not even be capable of human speech!) and uses this tactic to lure her into his clutches.
If the Mailer Daemon wants her to love/marry him, he will do his best to seduce her to The Dark Side with a Circling Monologue. If he fails, or isn't interested in companionship, the fangs come out and it's feeding time! (or chaos ensues).
This kind of episode usually ends in An Aesop about not trusting people on the Internet, at least not blindly, and not shutting yourself off from real people in favor of "imaginary" people (who are all stalkers) on the Web.
Other variations include using a Great Big Book of Everything instead of a PC, or having the Daemon communicate while being trapped in Another Dimension, and asking the heroine to use Summon Magic to bring him over for face to snout chat.
The trope name refers to "Daemon", a real piece of computing lingo. "Mailer Daemon" usually refers to an automatic process that runs in the background and handles incoming email messages.
If this takes for a short amount of time, it is a kind of Prank Date.
- A friendship sort-of version happens in the last Black Jack series, in which a Japanese Ill Boy lies to his Australian online friend about his prowess in baseball and breaks off the friendship in absolute panic when said friend says he'll visit him and watch his games. It turns out the Australian kid also was lying... because he was another Ill Boy, and actually blind. When they make up, Black Jack operates on both of them and they get better.
- In Corrector Yui, Grosser's subconscious met a very young Yui once, and when he became Grosser properly he became obsessed with her without knowing the exact reason why.
- In the very beginning of Durarara!!, this is basically Izaya's Establishing Character Moment. He uses the internet to be a sympathetic ear for a depressed teenaged girl named Rio Kamichika and talks her into a mutual suicide, then has her kidnapped and rescued, then tells Rio what he did, and then he tries to lure her into committing suicide for real by saying not even he cares if she lives or dies. All for shits and giggles. She's saved by Celty, but barely.
- In Cyborg 009, Francoise aka 003 gets one of these: the super-computer named "Sphynx" that controls the robot city of Compu-Utopia. That AI actually is the placeholder of the memories and mind of Carl Eckermann, a deceased young man with hidden and huge Mommy Issues, who happened to be the son and right-hand of the creator of Sphynx and Compu-Utopia.
- In the comic Ultimate X-Men, cynical teenage genius Beast managed to get involved with someone he really thought was an attractive, brilliant (and mutant) model online. The "model" turned out to be the remarkably clever Ultimate version of the Blob, out for nothing more than to humiliate him by showing up to a rendezvous and revealing the scam. Too bad Beast got too trusting and and let slip a dangerous secret...
- The 'true story' "There's An Alien On The Internet" from the first edition of Chicken Soup for the Kid's Soul is a subversion: a boy makes friends with somebody online who seems nice but avoids revealing anything personal at all, even the kinds of things that no stalker could ever hope to use against him/her. When the boy presses his friend for more information, he/she reluctantly admits to being a space alien and describes their otherworldly origin in great detail. However, it turns out that the friend is actually a perfectly normal kid living in another state who was ridiculed for being disabled and could only make friends with the anonymity of the Internet.
- In the book Evil Genius, the titular evil genius successfully pulls off one of these. He feels bad about it afterwards.
- Ginny Weasley and Tom Riddle's diary in Harry Potter and the Chamber of Secrets qualifies. The diary she befriended turned out to be a Soul Jar for a piece of Voldemort's soul, who Brainwashed her with the intent of stealing her life to bring himself back to life, taking advantage of how she was feeling lonely due to her difficulty to make any friends in her first year at Hogwarts.
- Christine only communicates with The Phantom of the Opera through her wall for 3 months, never seeing his face and only knowing him as "the Angel of Music." She tells Raoul that she fell madly in love with him, but she was also terrified at the control he had over her soul — she couldn't recognize herself anymore, did whatever he told her... When Raoul tries to tell her adoptive mother that she's in over her head with a guy she doesn't know, Christine gives him the familiar "You-don't-know-anything-about-him-it's-none-of-your-business" speech. Then, of course, he kidnaps her (drugging her to ensure her cooperation), leading to the infamous Dramatic Unmask...
- The Angel episode "Couplet" had a life-sucking tree demon with a DSL connection that lured men to their deaths by pretending to be a woman on the Internet and getting them to come meet "her."
- The Buffy the Vampire Slayer episode "I Robot, You Jane" had another twist on it, when Willow's cyber-boyfriend turned out to be a literal demon on the Internet, Moloch the Corruptor. (The magical book in which it was sealed had been scanned into a computer.) The metaphor was lampshaded when Buffy used it as an argument to persuade Willow to check up on "Malcolm".
- In Penny Dreadful both Dracula and Lucifer qualify as this while also being the Big Bad of the series. Both use their supernatural abilities to lure female protagonist Vanessa Ives to becoming their bride, and thus the Mother of all Evil.
- One episode of The X-Files featured a charming "fat-sucking vampire" who would lure lonely, overweight-by-Hollywood-standards women out to secluded areas to feed upon their fat cells.
- Odyssey 5 had "Kitty", an insane AI who developed an obsession for Neil. At first chatting with him and sending "pictures" of herself that never showed her face, but then getting clingy and jealous, eventually using her vast AI power to monitor him with cameras and control nearby appliances. She had to be tricked into a computer before being unplugged, essentially making her a Sealed AI in a Can.
- The Book of Unremitting Horror gives us the Sisterite. A modern day succubus, they use their powers to access the internet, and use dating sites to track down prey (the fatter, the better) and begin a rapport with the victim, luring them into a false sense of trust (or at the very least, make them too horny to think straight), often supplementing this by emailing a .jpg of a sigil that not only makes the victim see the most beautiful woman they can think of, but it allows the Sisterite to further twist the victim's perception at a later date. Like making that abandoned barn look like a dance club. Perfect for when she's ready to meet "In the flesh".
- This is the main premise of RE: Prince of Nigeria.
- In a lengthy storyline in Sluggy Freelance, Gwynn is seen conversing with a friend on the Internet: a friend who happens to be skilled at the very type of black magic she's been trying to master. His advice always leads to disaster (of the kind where Hilarity Ensues, but still...) but she never seems to notice, or blames herself for not being careful enough. It's only after she's finally gotten fed up with her distrustful friends, and left on a bus to meet her 'Internet friend', that we find out that Zoe's Internet account - which Gwynn had been using - has been down for ages, and the 'friend' was actually a shard of K'Z'K the Vowelless, embedded in her brain, making her type his half of the conversation out in a word processor. He just needed to get her away from her friends so he could extend his influence to a complete possession...
- The SCP Foundation has SCP-1269: A mailbox. A very, very, possessive mailbox.
- Soren Bowie parodies this with his article "The Spambot Who Seduced Me: A True Story of Forbidden Love", in which he responds to actual messages sent to him by a dating site spambot (also mocking how common these spambots are in the comments on Cracked articles), with each reply being more and more lovesick.
- Futurama: In "A Bi-cyclops Built for Two", the Planet Express crew is on the holographic Internet when a male cyclops approaches Leela. Up till then she had never met another cyclops and knew nothing about her origins as a sewer mutant, and believed herself to be an alien. "Alkazar" persuades her to come to their "home planet" where he tells a tale of how they're the last survivors. It all falls apart when Bender and Fry find 4 other alien women on the planet and reveal the man to be a shape-shifting dude who was scamming his way into "making it with 5 weird alien babes".
- Raven from Teen Titans fell in love with Malchior, an Evil Mentor trapped in a Tome of Eldritch Lore, this way (fittingly, he's also the page image). He used her introversion, loneliness, and annoyance over her goofball teammates to sway her into learning the spell necessary to release him. Oh, and by the way, he's also a dragon. Malchior also managed to swap his name with the hero's name (who sealed him inside the book to begin with) so Raven wouldn't deduce anything wrong until she finished casting the spell.
- There's a particularly nasty version of this that happens in Real Life, called the "romance scam". The scammer pretends to conduct an Internet romance with the victim, then tries to convince the victim to send them money so they can meet in person, or because they're in trouble, or for any number of other reasons.
- This is a variation on the "Spanish Prisoner" con and goes back to long before the Internet. Then it was done with written letters, usually hand-passed to a 'close friend' (the con artist) for delivery to the 'lover'; some con artists used to have several dozen victims on their line, all sending money and other valuable (and resellable) gifts to their 'one true love'.