Leona (Orochi) Heidern, Orochi in the middle because she was 'technically' Orochi before she became a Heidern.
Anyway, just to kick things off, if I had to make SPECIFIC tier listings, I would say that it goes something like this:
1-Athena
2-Choi
3-Billy Kane
4-Leona
5-Vannessa
While the others are subject to debate as to order, it is near undisputable that Leona is definitely one of the higher tier characters, due to her solid air game, great ground game(low pokes, overhead, projectile(sort of), *INSANE* combos for her type of gameplay), she lacks nothing and has a lot left over from certain areas(range?!).
While Choi lacks defense, Billy Kane lacks offense and Athena lacks in general comboability with her moves, Leona has none of these problems...
Anyway, Leona has no standard combo, no 'typical' bread and butter 'one' to pull off constantly. She can do near 100%(near), starting from a stand D, it depends on your stocks and if you want to lose health(riot of blood mode).
Her basic poke combo is :
-Crouch B, crouch A, down-up A(or C I suppose)
It does more damage than it would seem, especially in riot of blood mode.
An impressive fact that I just learned today is, that Leona can link her V-slasher from her jump A which is pretty quick to come out , which is good to surprise enemies, I'm certain we *all* know that she can link the V-slasher from her jump C/D, and if you didn't, well , now you know

While Leona has some lovely combos and great low attacks which are 'exentuated' by her knock down overhead kick, her game mainly consists of poking and zoning.
It is worth mentioning that in 2002, V-Slasher will outright defeat ALL aerial attacks, at least every single one that I've seen. Making it, hands down, the best air to air 'move' in the game.
Riot of Blood mode activation is "u,d,u,d,u,d + BD"(for short). This mode automatically drains half of your health, BUT, it removes your 'attack' recovery, in the same way that max mode activation does, thus this allows you to further 'continue' some combos.
Riot of Blood leona, while being the fastest character in the game, does not possess as much power as one might imagine, it is not advisable, you lose your ability to high jump, thus, losing a lot of V-slasher opportunities, and of course, the health thing.
Leona is high tier, due to her great zoning abilities, arguably overpowered V-slasher priority, lovely pokes and (arguably again) 'broken' combos, truly, unlike Athena, Billy Kane and Choi, she lacks nothing.
Discuss Leona !