We never know about Sougo's parents, but in episode 25 our question has been answered.

Sougo's parents died at April 24th year 2009.

It looks like Hiryu Kakogawa aka Another ZI-O know Sougo.

Tsukuyomi ask where is Sougo's parents to Junichiro Tokiwa (Sougo's uncle). He said that his parents died in accident, but there are only two survivors survived from the car accident.

And that survivors is Tokiwa Sougo aka Kamen Rider ZI-O and Hiryu Kakogawa aka Another ZI-O.

Could it be that Hiryu and Sougo are BROTHERS!!!!!???!??!!??!?! 😲

Hiryu Kakogawa aka Another ZI-O's main goal is to defeat Tokiwa Sougo. I believe that is the reason why he became Another Rider. But how did he able to transform into various Another Rider ? Well the answer is simple, he is taking the power of all the previous people that contracted to be Another Rider.

And those peoples are this:
-Unnamed Basketball Player (Another Build)

-Ida (Another Ex-Aid)

-Ryuichi Sakuma (Another Faiz & Fourze)

Hiryu also obtain the the form of Another Ghost, Gaim, OOO. Im pretty sure Hiryu also target King Dan Kuroto to gain the form of Another OOO.