Saki (manga)/WMG

Hiroe is Hisa's estranged former Middle School teammate.

 * First off, start with her sneer that was directed towards Hisa when they first appeared alongside.
 * Look at the color pattern on Hisa's Middle School uniform, and then compare it to the color pattern of Hiroe's uniform. You'll see that they're extremely similar.
 * Based on this, it would be safe to assume that the team Hisa played for back in Middle School must've been some sort of feeder school to Himematsu.
 * Not to mention that when Hiroe takes special note of Kiyosumi High School, Hisa's face is the first thing that pops into her mind. That, and her reaction to Hisa completely wiping the floor in her match is muttering to herself about how she and her team ought to dish out a royal ass-whooping on them.
 * All that considered, it can be inferred that Hiroe might have been Hisa's teammate in Middle School, and is seeking to pay Hisa out for withdrawing from the Nationals in Middle School by preventing her team from winning a championship, just like Hisa prevented their school from winning one.
 * Jossed as of Chapter 77.

Hisa withdrew from the Nationals in Middle School because of her parents' divorce.

 * Her name changed from Ueno Hisa to Takei Hisa after this...Not much to really explain here, is there?
 * If this is true, it will explain why Hisa is very serious about getting Saki into the mahjong club. She sees a little bit of herself in Saki, and don't want the latter's family to break permanently like hers.

Jindai pushed herself too hard in the previous year's tournament fighting Teru.

 * In Chapter 56 of the manga, Hatsumi mentions how all of the Kagoshima family branches have sent representative players for this year's tournament for the purpose of helping Jindai, who had pushed herself too hard the year before, and is already rather frail. Jindai serves as the Vanguard (First player in a team tournament) for Eisui - The very position that Teru plays for Shiraitodai. And since Teru is heralded as the world's Number One player, and an extremely powerful opponent to boot, it doesn't take much brainpower to infer a causal relationship from here.
 * And, on a side note, look at how similar Jindai looks (Especially in her hairstyle) to the way Saki looked as a child...Hmmmm,

Saki can reunite her family through Mahjong

 * In a Heart of the Stones way. Seriously, with all the Super Powers around, we are definitely in that kind of setting.

Hagiyoshi is a ghost.

 * I don't think it needs much explaining as to why I think this.