IGA was the Writer as far as HoD, Aria, Dawn, Lament, and Curse.
For Portrait there was
IGA
Hiroto Yamaguch
Shutaro
For OoE,
IGA,
Tomohiro Takeda
Katuyuki Kuriyama
Takayoshi Kakehashi
For LoS,
Enric Álvarez
Dave Cox
Eddie Deighton
Jon Sloan
Enrique Ventura
Luis Miguel Quijada
Notice Cox is second on the list- (in PoR, OoE, and even Symphony, IGA is always listed first) Alvarez seems to be the main writer for LoS. And since there's a whole bunch of em, they all just throw ideas into the pool and eventually have to come to some sort of consensus. Which is then added into whatever Alvarez plans to do. Cox may be a Castlevania fan, that doesn't mean the rest of them know anything about Castlevania. The few Castlevania elements could be courtesy of Cox himself while the rest is the other writers trying to interpret Castlevania.