Thinking about Reiji Sugimoto again because I like his design, so I'm just gonna list a few head canons. Will they change later? Probably
Between Goto Ryuji and Reiji Sugimoto, I think Sugimoto is the more laid back one based on their phone call. Sugimoto might be the ones who let the other S-Ranks do what they want while Goto is the one to reel them in when they get too energetic. That's why Goto looks like a tired dad
He might get along with Cha Hae-In when they meet at the International Guild Conference. Might exchange some juicy gossip :3c It's probably mostly Sugimoto sharing fun little stories, but Hae-In would eat it up and share a couple of things about her guild too
After some thinking, I've concluded that he is most likely a mage. He doesn't have the build of a tank, so I eliminated that option. As for a healer, all the healers we've seen so far are shown to have a cheery and optimistic and outlook on life with a strong desire to help others. While he is optimistic, he doesn't have the same energy as other healers, so I eliminated that option as well. As for a fighter or assassin, he's not shown to have any weapons, which could be an art choice, but it is unlikely as the other Japanese S-Rank fighters/assassins have some kind of weapon (Kenzo Tanaka being the exception as it's clear he fights with his bare hands). Mage seems like the most likely class as he has the same kind of thinking as other mages (Choi Jong-In and Kei) where they observe and use critical thinking to assess the situation before acting. It would also make sense for him to stay behind if he was a mage since he is more likely to work better in a group and be in the back rather than fighting up front (Kei has both defensive and offensive skills, making him an exception)
As for what kind of mage he is, I think he could be a support mage. It would be another reason he would stay behind and why his skills might not be talked about or shown as much. He could also be an offensive mage, but I'm not sure what kind of powers he would have. I'm open to possible theories about this though
I'm think he's around his late twenties. He doesn't seem like Cha Hae-In's or Sung Jin-Woo's age (early, close to mid twenties), but he also doesn't seem like Goto Ryuji's age (old). He might be around Kanae Tawata's age, who is 27 according to SL: Arise. I like to think he awakened at an early age (late teens) and when the gates first appeared, so he matured at an early age but still has a bit of whimsy in him that comes out around younger people (most of the Japanese S-Ranks and Cha Hae-In). He might also be on the more social side since he's usually shown to be speaking with someone in the manhwa. I'm imagining him like Jong-In but more carefree and social. I get fox energy from him for some reason. Might be the eyes
I just like think Cha Hae-In gets along with the Japanese S-ranks a lot and keeps in touch with them :> She would feel more relaxed around them and not like she has to be wary of how others will judge her as Korea's only female S-Rank
