I think the closest word would be , although the meaning might be a bit different.
refers to the person the women is confessing her love by giving him chocolate at Valentine. It's probably close to the English word "Crush", although it is implied that the woman is actively trying to get together with him.
As for the phrase `Won't you be my valentine?`, I think `` could be the closest. Note however that this sounds more juvenile compared to `Won't you be my valentine?`.
These special chocolates are called and are distinguishable from by how fancy it is, how intricate the wrapping is, what the content of the accompanying message card is etc. (Just about any Japanese male can write a book on how to distinguish from ;)
For most Japanese women, valentine day is pretty much the only occasion where they can confess their love to men (instead of vice versa), so it's very significant. Japanese teenagers are particularly obsessed with in Valentine season.