When a fictional character enters an officially recognized relationship with your waifu
, thus making her unavailable, he (or she) becomes a waifu stealer. Usually, the character is described by fans as a horrible, mean person or even a womanizer
, even if the character hasn't shown any signs of being that way.
The term first came to real prominence in the brony
community after the first My Little Pony movie was released. In it, Flash Sentry, an incidental character horns in on Twilight Sparkle, a main character, thus pissing off any of the bronies who called her waifu.
Sometimes also referred to as "waifu thief".