From your list, it sounds like Rogue is the best fit for this character... Unless there's something about Rogue in particular you think doesn't fit.
Well, fully aside from flavor issues, the main problems with Rogue are:
1) he would never attack a foe from behind (aka flanking) so he'd be fairly mechanically gimped by missing out on one of the rogues main class features (sneak attack).
2) he wielded his katana 2 handed, so the best approximation is a bastard sword or even greatsword, not a rapier.
3) the "flavor" of rogue powers is even worse than the flavor of avenger powers (sneaky, dirty tricks as opposed to channeling divine energy).
4) skill-wise, rogues are worse than avengers too - he certainly was never trained in stealth or thievery... and 4e rogues are also missing diplomacy from their skill list.
No, I think avenger is the best fit in 4e at the moment, even if it isn't ideal. Maybe if/when a swashbuckler-type character is released (presumably Dex/Cha), it'll work better.
Dragon Blood Sorcerer is a good option if you want to increase his strength.
As I said, him being a weakling was fairly central to the character, so this isn't a good option.
Otherwise, be a Chaos or Storm Sorcerer, but take mostly "Dragon" powers.
Again, this mechanically gimps the character, as he won't be able to benefit from the strength rider effects of those "Dragon" powers.
Otherwise, what was his shtick in combat?
If I had to sum it up in a word "acid". If I could add another word "draconic". His main spell was acid orb, he had acid breath and acid sheath (and obviously acid splash). He had the acid reserve feat from Complete Arcane. His secondary spells were all dragon-like - dragon skin, wings of the dragon, that sort of thing.
Unfortunately, I just don't think there is a good 4e analog. This character did end up dying, so the conversion was purely a hypothetical exercise. I was just curious whether 4e could do my favourite 3e characters. Apparently not.