Jin does seem very similar to Ky.
They have the same color scheme, the same fighting style flavor-wise (swordplay with element attacks), the same role in the story (representing the overbearing government, opposing the renegade main character) and the same gameplay outline; very similar projectiles (which they use heavily), decent reach, pretty straightforward without a lot of crazy gimmicks like the rest of the cast.
What seperates them besides their personalities?
EDIT: I think I should elaborate a bit.
If Jin was in a different game it would've been fine since nobody would immediately try to make connections between them. But when you make a game that's effectively the followup to Guilty Gear, the first thing you think before looking at the new main characters is "hmm I wonder how the new characters compare to Sol and Ky." Then you get Ragna and Jin, it's kind of like...yeah...
I'm pretty sure Jin is supposed to resemble Ky, but there are still differences in styles, otherwise people would just go play GG.
Ky's projectiles have a lot less recovery on them, so they can be used in blockstrings, and FRC'd if you want to tick throw or something, zoning, etc. Jin can still do all these things, but he either requires more meter to do it (since there are no FRC's in BB), or he may end up at a disadvantage due to his relatively slow startup, since his projectiles cannot combo like Ky's can with basic gatling to Stun Edge.
One thing Jin can do that Ky can't do (at least, in AC) is deal nice damage without using meter. IIRC, Ky can do about 35%-40% using 25% meter, and maybe 20-30% without. Jin can easily do 30-40%, but most of his high damage combos start from 6C, which is fairly easy to block. He can still combo off things like counterhit 623C, but it does far less damage than a decent 6C combo. Jin can also do decent damage off a throw, but that's more a difference between GG and BB than Ky and Jin.
There are also differences in their movesets as well, such as Jin's 623 compared to Ky's. Ky's Vapor Thrust is usually a combo ender along with his lightning javelin, and hardly ever used for anti air. Jin's Fubuki/Rehhyou is designed for anti air, so it's either quite fast on startup(623A) or has really good invincibility(623C). Jin's 214 (aka ice cars) is also enough to make Jin different from Ky. Ky's 214K (Greed Sever) is a simple overhead that can go into a combo, but Jin's is a fairly reliable source of transportation in the air (escaping from pressure with j.214C, etc.), and a combo ender.
Summary: Jin is similar to Ky in design, but in battle, he's got enough differences to make him play much differently. He's also insane.
Edit: Or what Letty said. That works too.
