What I don't understand is why people see them as Visual Kei in the first place.

They don't have crazy hair or make-up or costumes.

They just look like well dressed regular business men (minus Tetsu who looks like an artsy person but still stylin'). The clothes they wear is the same type of clothes regular people wear.

VK on the other hand..wear costumes. Things the average person doesn't wear. Dramatic make-up and often times rainbow hair.

L'Arc doesn't look anywhere close to VK. They don't even look like a rock band. They looks like clean cut but stylish business men.
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When Hyde mentioned fans liking them looking pretty I think he is right. Many female fans like pretty bands. Look at all those VK bands that have one screechy album (or haven't even released an album yet) and have fans. And then other bands that aren't all pretty but have decent music get less attention. (Heck on online fandom it seems that VK bands get more attention than L'Arc/VAMPS)

Of course, I don't think that's the reason people are fans, it's just an added bonus. L'arc is pretty, clean cut, and professional. The type of band your parents are okay with you listening to.

VAMPS is more.. grungy, dirty, cheap, and raw. The kind of image that would make some parents say "Wtf? sex blood rock n roll!? You're not listening to this anymore!" -burns cds-

But I kinda like how VAMPS has a kinda naughty image but at the same time we all know they're not that bad. I highly doubt the guys are snorting cocaine of some model's butt, drowning themselves in booze then passing out in the trashcan. They look kinda rough (to people who don't know better) but are actually good. They have a nice balance between clean and rough.

(Edit: When I say they look rough I think you mostly see it in concerts. Interviews and stuff they look normal. But concerts they looks really grungy, rock n rollish while L'Arc is all professionally styled and clean)