To be serious about waifu, since I consider all the previous posts, including my own, lacking in any substantive explanation, I'll do what I can to define what a waifu is.
On the most basic level, a waifu is a female character that someone feels a certain emotional connection to.
It is essential to any story that the readers or viewers are exposed to characters that they can connect to. This is where my interpretation of the definition of waifu comes from. As I'm watching anime or reading manga... I may find a character that is so well developed, that even if I can't completely relate to them, as the story progresses, I feel a certain emotional attachment.
Bleach fans can understand what I'm talking about, since major characters are dying. Not to mention how ChÅjirÅ dies, the revelation of his past with Captain Yamamoto, and how Yamamoto gets fucking serious.
We all love anime and manga, so if you say you haven't become emotionally invested in what happens to a specific character, you're full of shit. Anyone who was around for the MoteMote competitions knows that I'll post a wall of text to stand up for my waifu. By the way, when the fuck is there gonna be another MoteMote competition.
This might not be the right place to say this, but I'm not sure Data Zero is worthy of having Misato as his waifu, but we can get into that another time.