There is a valid argument for each of those characters, and I'd assume the japanese players know more about this game than I do. However, for the sake of theory fighting, which I find enjoyable, I'll give my input as to what I think the tiers are, in horizontal order as well.
S: Sion, Akiha, Len, Ciel
A: Aoko, Kouma, Mecha, Hisui, Sacchin, Hisui, Warc, Nanaya, Vakiha
B: Miyako, Arc, Maids (Kohaku lead), V.Sion, Tohno, Nero
C: Kohaku, w.Len, Maids (Hisui lead), Wara
Neco's
I feel it's necessary to separate Hisko and Kosui as they are basically different characters. All the S, A, B characters are very close together compared to tier lists of other games, with only the C and D characters having a universal disadvantage in most matchups. I guess what I'm trying to say is I'm very happy with Melty's balance as a fighting game.