OK, I got ahold of super brother. Here's what he wrote up:
"Re: RO stats. For the example of physical defense stats;
-Vitality increases HP and applies a straight numerical reduction equal to roughly 80-100% of your Vitality off of every hit.
-Agility increases your chances of evading an attack by 1% per point -- which is directly countered by Dexterity, which increases hit chance by 1% per point, and by level, which increases both hit and flee by 1% per level.
-Armor applies a percentage reduction to incoming physical damage, and elemental and attack property-related modifiers (ranged, melee, etc.) also apply percentage reductions.
-Luck also plays into defense, granting Lucky Flee, which is a straight,
non-reducible percent chance per ten points to dodge incoming damage, and also reducing enemy critical chance at a rate of 1% per 5 points (which is lower than the 3-1 ratio for gaining crit, but still contributes to a rounded defense).
(Armor can also have magic defense, which is percentage-based like armor's physical defense, and both intelligence and vitality reduce incoming magic damage numerically.)
With defense spread out like that across the board, a lot of interesting builds can be viable if played and built properly -- for example, an Agility-based Wizard is a surprisingly effective character, and that built caught a lot of players on aRO completely off guard. With the fact that higher stat totals cost more to increase than lower ones (a stat below 11 costs only 2 stat points, while a stat above 90 costs 11), it ends up being important to make a character well-rounded rather than solely devoted to a single stat. Essentially, every character should have at least small investments in every stat to be maximally effective (roughly 10-20 points in every stat except Intelligence for melee fighters and Strength for casters, with larger investments in whichever stats benefit your build most)."
So, actually, 5 defenses. Whoops!
It bears mentioning my brother was only interested in the PVP castle siege gameplay in RO, and he was real good at it. Dunno if anyone here was playing at the time, but later in the server's life he held absolutely every castle on the server for a couple of weeks (which sounds more awesome than it is, since castle siege was only held weekly -- but still). He also described the PVE gameplay as "totally indefensible," in part because the math breaks down -- there's no confusion about how enemies attack or what kind of damage they do, so using an out-of-canon build doesn't do anything but reduce your effectiveness, unlike in PVP where FLEE WIZARD fucks everyone up until they figure your tricks out.
The clever thing is that, once you know what you're buying for your points, you can get away with unusual things that work better than you think, allowing some freedom of choice in character growth! Contrast with WoW, where i can look at someone's talent trees and instantly tell if they know what they are doing or not -- there's no room for experimentation, personal style, etc.
