The OP has said he KNEW he was supposed to carry the card. He chose not to. Fear of losing it is not an excuse. There are procedures for reporting it lost and having it reissued. If you report your card lost and get stopped by the cops on the way to get your new one, do you really think you would be rousted for not having the card? Not likely.
Actually that was what I told them at the police station, because I thought it was the best thing to say.
Originally I did know that carrying the resident card was necessary (after all of this happened, I remembered being told at the airport), but it slipped my mind... or more precisely, as I have already said, I just thought that the ID card was enough since my name, picture, university and period of study are written there.
I was careless, I've admitted that already. My bad.
But the "not carrying documents for fear of losing them" is still true for me and I often prefer not to carry many things with me for this reason.
He has shown what is a far too common trait among many foreigners: that of being a scofflaw. Apparently he wasn't afraid of losing his student ID, as he carried that with him. So the "I was afraid I might lose the gaijin card" thing doesn't hold water any way you look at it. He just chose not to carry it, despite knowing it was required of him. He chose to have his brother live with him, despite there being a strict prohibition against even visitors. He follows whatever rules he feels like, when he feels like it, if he feels like it, and only worries about the consequences when things have already gone wrong. This is all ナスツゥ窶ケテ?スツゥ窶慊セ (brought on himself, by his own actions), so it is hard to work up a whole lot of sympathy for him. He had to learn the hard way.
Actually I carry my ID card with me because it's necessary to get in the dormitory. Despite this I'm still afraid of losing it, which is anyway exactly what happened about a month ago: I put it in my pocket and left the dorm, then when I came back home it was not there (oh, how I wish I had your invincible pockets, WonkoTheSane)
However, having it reassued was pretty easy, I just had to ask the student division and to wait a week. But I've been told that losing the resident card might take a little more than that.
I don't remember having asked for simpathy. I just asked for help because I (wrongly?) thought that what I did might have had nonproportional consequences (criminal records? Fine? ...deportation? :\).
Fortunately none of that happened (criminal records aside maybe, still note sure what all of that entailed...) since I'm not a criminal and I was not challenging the law. I am only a 22 years old student that was going to buy some manga at a book off who left his documents at home (by mistake, not to challenge the law because I feel I have gaijin powah or something).
Anyway, we have been stopped again with the bikes (for the light thing) and they asked again for registration. Since I have documents of one bike and I know the owner of the other one, in both the cases there was no problem at all.
Oh, I remembered one last thing I would like to add. In April, the first month I arrived here, I was stopped by a police cop while a was at Shibuya for a document check. That time I was not riding a bike but simply walking. I don't know if this is normal or not, but thinking about it now (after this mess) is a bit more annoying than when it happened...
Luckily I had my resident card back then!