I think by itself, this is a really hard question.
First and foremost, one has to venture a definition of
imouto. Of course, the literal meaning of it is 妹. The kanji for it is comprised of 女 and 未, which mean 'woman' and 'not yet' respectively. From there, one can easily see why it's used to say 'little sister'. But owing to the fact that it's a loanword used exclusively by the various Japan-related fan circles, it does not carry such a literal meaning in English. In the Japglish vernacular,
imouto carries a special meaning, much like
otaku or
tsundere. So what does this all mean? Does the
imouto label not apply to my 13 year old sister, even if she is, literally, an imouto? Yes. It's rather hard to grasp at first, but it becomes easier with time.
So now that we have established a clear meaning of this loanword, we can then move on to the next point. What makes an imouto, an imouto? In the opinion of this writer, there's too much and too little to go on, at once. For one sure thing, the
imouto has to be a few years younger. A few years is absolutely essential. A 16 year-old little sister of a 17 year-old boy isn't an
imouto at all, she's just a regular sister. However, paired with a 22 year-old man, she fits the description just fine. An
imouto's personality has to follow a strict guideline. She's stubborn, defiant and yet violently coquettish. She instills in a sibling the feeling of endearment. The
imouto never fails to instill a sense of pride in her brother. He wants to show her off, not because she's attractive, but because she's his sister. An
imouto can banter, sometimes to the point where it turns into a fight. But she will never be an embarassment.
There's some debate, in the AKB48 circles. Why are some girls
imoutos, while others aren't? This is hard to answer, probably because this answer changes with every person. It's not really something that can be explained; it is innate. One can determine who
isn't an
imouto, but not one who is. At least, until one meets a girl who qualifies. "What about Myao? Everyone agrees with her!", you say. That is true. But this is really because Myao exhibits a universal
imouto-factor that everyone recognizes. It is not tangible, but it can be abstracted and perceived by everyone around her. What about girls like Komori Mika then? She fits everything, yet is not seen as an
imouto. This is probably hindered by the fact that Komori is not very assertive.
Imoutos assert themselves onto whoever they're up against, be it their brothers or parents. Komori, while ostensibly having all the qualities of an
imouto, fails to do so, due to her lack of personality. This does not mean she is unattractive or otherwise, it just means that she's not an
imouto. Now that the explanations are in order. Who would this humble writer pick as his ideal imouto?
Disclaimer: I actually really love my sister. Don't tell her I said that though.