Overall TenSura S1 is great. It sets a lot of foundation for the world with awesome word building between the different monsters, the ritual of the summoners+ sumonnies, introducing Demon Lords as a real threat and creates are very likable cast. It serves as a great first isekai for an anime fan.
The extra episode of TenSura S1 surrounds Shizue and Noir. This episode is mainly to showcase how the encounter between these two leads up to his encounter with Rimuru. Not exactly the most needed episode to watch, but you get more lore and world building attached. As Demon Lord Leon mentions to Shizue he is a Primordial Demon, whose strength far matches that of an Archdemon.
The rest of S1 centers around Rimuru saving the kids. Which requires him to seek out the Queen of the Fairies. Who also turns out to be the Demon Lord Ramiris. The process basically has them getting a superior spirit of sorts to inhabit the body of these children like Ifirt did for Shizue. Rimuru is able to aid the children in getting this done except for Kenya and Chloe. The one that leaves an interesting mark is Chloe.
She gets a spiritual body which is the equivalent to that of a superior spirit. But given that Ramiris has a bad feeling when this one comes out, initially there is cause for concern. Though when Rimuru sees the spirit moving and in light is reminded of the hero that saved Shizue, it could be this spirit inhabited the hero? Or this is the hero in spirit form. When you see Rimuru give Chloe Shizue's mask based on feeling, you get the sense that Chloe's path is parallel to that of similar to the hero and Shizue.
EP 3–5 centers around Rimuru, the school and his students. But separating Rimuru from his students, it allows the kids to set center stage. When they encounter the bandits in the cave, you get to see the potential of what each could be in the future.
Coming to their Triss-sensei and the kids' rescue is Noir. Unlike in the extra episode, his cost of being summoned is only to maintain his secret. He serves to repay his debt to Shizue, which further serves the point of him and Rimuru's paths are destined to collide as mentioned in the extra episode.