fu is the phonetic way a japanese person would say an English 'f' sound. Waifu was origionally just the phonetic way some anime character referred to his wife (wa-i-fu), because he was a creepy dude and the picture was of a beautiful woman it memed into meaning "the person I pretend is my wife" or "my imaginary wife". Daughteru is a retarded way of only using the phonetic translation at the very end of the word. If you gave a fuck about the phonetic pronunciation it would be Da-ta or Da-ta-ru, but what people are mostly doing is a play on waifu which would make it daughterfu because it makes the most phonetic sense in English.
As a former weeb it pisses me off that people are trying to shoe-horn part of the phonetic japanese without going the full mile, and then calling people who do the same thing wrong. Daughterfu makes more sense to English speakers, Data or Dataru would be the best fit for fags who care about japanese phonetic translations of english words, and daughteru is retarded.