I binged the manga a while back, but I'll try to recall. The protagonist is Masaki, a guy who gets reincarnated into a fantasy world with his entire office building. He's pretty chill and uses his modern knowledge, plus a bit of cheat ability, to build a settlement. The main heroine is definitely Leonora, the elf girl he saves from slavery early on. She's a healer and becomes his right hand, managing stuff.
There's also the beastkin girl, Siera, who's a fierce fighter and joins the community. The dwarf blacksmith Brokk is a big part of the 'kaitaku' or development side, making tools and weapons. The cast grows as they recruit more people, but those four are the core group you follow from the start. Honestly, I sometimes mix up some of the later side characters who show up for an arc.
Masaki's whole deal isn't about becoming overpowered and fighting a demon lord. It's more about nation-building and logistics, which I found a nice change of pace. The focus is really on him and Leonora's relationship as they build their new home.