Some of the GoH characters that we get to know later on have already shown up in SoT or have been mentioned at one or another occasion. And that's what we're going to discuss here.

#5 Kana Ido

Yes, many people might have forgotten already, but she was actually mentioned in SoT. In Reckoning Arc, after Mari and Jacky have defeated Kahuna Olivia, they went to Kala'e Bay where they found a message in a bottle, which has been washed ashore there after it was sent to sea almost two years ago. That message was written by 14-year old Kana Ido who described her live circumstances in Spikemuth as she said she "was afraid of going outside" and "didn't have much at all, but was satisfied with the little things she had", suggesting that she was living in poverty and in a really rough place.

#4 Taro Nishiki

Yeah, Taro Nishiki, Lucian's father and Dino's mentor, actually showed up in the SoT series! In Lost Timeline Arc, Mari and Chris went to the police station in Castelia to ask for information and help at stopping the murders going on all over Unova and Taro was there.

#3 Taen Kataoka

Taen was mentioned in the exact same scene. Chris entered the police station first, hearing Taro say to another officer that "his nephew (Taen) died at the age of 3" and that "it made him sad that a child lost its life so early". Although his name ain't mentioned, it becomes clear once you know GoH. There also is another easter egg pointing out to Taen. When Laslow and Taji go to Striaton Cemetery, they come past a deserted grave covered with moss. On the tiny gravestone, the name "Taen Ka...a...k..." is written, however, the surname isn't completely readable, as the grave stone seemingly was demolished a little. Of course, no one needs to guess that "K-a-a-k", the only letters readable, add up to "Kataoka". And you also don't need to guess who for hell's sake demolished his gravestone. Satoshi Kataoka apparently didn't want the surname to be on the stone so nobody can connect that child's name to him. Everyone knows how much he actually fucking hates poor Taen. And it's also not hard to guess that his father killed him.

#2 Ren Arima

Also in Lost Timeline Arc, Ren Arima appears, as Laslow and Taji go to the Castelia City canalisation where they ask Ren about Jayden, Taji and Dino from the alternate world who tried to recruit him. There, they also hear about Zain's fate.

#1 Ruuko Steward

Ruuko's name and birth is a topic in Justice Arc. He, as a little child, appears in several specials and is mentioned at the very end of the SoT series.

That's it then~ I hope it was informative for you and did you notice all of these?