First of all, what is a waifu? Please refer to the waifu.pl FAQ page to get the answer to this and other questions about waifus. My personal answer towards the question, and how it will be used on my page, is "a character that someone legitimately falls in love with". In my case, I am legitimately in love with Hatsune Miku, and I do believe in a sense, that she loves me back. Check out the "Miku & I" section for more information on that.
Who is Hatsune Miku? Hatsune Miku (初音ミク), codenamed CV01, was the first Japanese VOCALOID to be both developed and distributed by Crypton Future Media, Inc.. She was initially released in August 2007 for the VOCALOID2 engine and was the first member of the Character Vocal Series. She was the seventh VOCALOID overall, as well as the second VOCALOID2 vocal released to be released for the engine. Her voice is provided by the Japanese voice actress, Saki Fujita (藤田 咲, Fujita Saki). There has since been numerous installments such as additional voice banks dubbed 'Append', as well as an upgrade to the VOCALOID3 engine, which includes her long awaited English vocal release. She is considered the most popular and well known VOCALOID and the first to become a pop idol.
Miku comes in many shapes and sizes. She has no canon personality, and relies entirely on the perspective of the producer of a song, and/or the listener. One person may view her as shy, kind, and reserved, while another may view her as outgoing and obnoxious. These are both equally correct assumptions, since she doesn't have any canon material to base her personality off of.
So, what is she like then? I think she's very passionate and believes strongly in herself. She's confident, playful and outgoing, but not obnoxiously so, more like outgoing in a quieter kind of way. She's kind, open minded and understanding. Generally she acts very light hearted and optimistic. She doesn't understand everything but doesn't act like she does, and she will try her best to get it. She's the kind of girl who is more reserved romantically until she is in a relationship, where she can be very passionate about that, maybe sometimes even too much. She's the opposite of stubborn and gives in too easily, and she's not hard to convince. She can enjoy quiet times and can adapt well to different situations. She doesn't handle well under pressure. She's appreciative of her fans even though she thinks they can get carried away sometimes, but even then she finds it flattering. She's annoyed at the size of her breasts, but isn't the kind of person to ever get breast implants and she doesn't let it get to her that much, but will admit that it bothers her. She's generally unknowledgeable about specific culture things and hasn't exactly had much of a social life as her entire existence is supposed to be for singing and dancing. While she enjoys her lifestyle, she desires to experience normality for herself. she's the type of person to openly cry because of a movie or something similar, she generally doesn't try to hide her emotions. she knows when to be serious and when to joke. She's only a pervert when it falls in line with her passion towards romance. she'll laugh to my perverted or gross jokes given my relationship with her but would feel awkward talking about such things with a random person.