Communication is the key to any relationship, so it is no surprise that a romance can begin when your only contact is by communicating. This trope describes a relationship built upon characters sharing their secrets and flaws by some sort of long-distance communication; letters, emails, linked journals, etc. While one character may assume that their new love interest is in some far-away place, they might be as close as next-door. Rather than meeting in-person, however, their relationship begins like Pen Pals. The reasons why they don't meet in person can vary wildly.
One possible reason is that they haven't noticed or already don't like each other in-person. The identity that they've formed through the letters/online is completely different, or at least began with different assumptions about their behaviour. Talking to each other creates conflict, but sitting down to write in your magical diary allows for the feelings to flow smoothly.
Another reason for the relationship may be that one of the characters is trapped somehow, and cannot leave the place they're in. While the "trapped" character might just be a shut-in, more fantastical ideas (such as an AI in a box, Time Travel, Another Dimension, etc) are all possible starting situations. When the lovers are in this situation, the "trapped" character needs the other one for their freedom. The more supernatural the "trapped" character is, the more likely the romance was faked in order to manipulate the other character.
A popular reason for the characters to meet via long-distance communication is that one of them is visibly "other". The monstrous character knows that they can never be accepted by society, but by reaching out through letters or chat rooms, they can adopt a mundane persona. This online presence (or magical equivalent) allows them to find love with a "normal" person, who would otherwise find their appearance too horrifying to see past.
Actual examples of this trope tend to mix-and-match possible reasons, although a common element that they share is the surprise and feeling of betrayal during The Reveal, when the characters meet in person. The character who feels betrayed will be angry at what they might at first assume was a deliberate deception. A story might have a one-sided reveal occur early on in order to allow both characters that moment of betrayal, feeding the guilt at the character who did intentionally deceive the other one. Resolving that betrayal depends on what direction the work leans; comedic works will have the misunderstanding resolved and the characters continue the relationship, while tragic works will have one character intentionally tricking the other for some personal gain (possibly including killing/eating the other character).
Subtrope of Pen Pals. If this takes for a short amount of time, it is a kind of Prank Date.
