You totally can use Meshcore without repeater, and companions can be used like routers. This functionality can be enabled any time using one checkbox in app. It switches the radios to slightly different frequency and enables repeat mode in personal nodes, this gives you one small network. No repeater needed. And in this mode you don't clutter "main" mesh with your local mesh.
The main difference between Meshcore and Meshtastic is how telemetry is handled. In Meshcore to get telemetry the other party needs to request it, whereas in Meshtastic telemetry is sent in flood mode in configured intervals. That's why Meshtastic is better suited for (A)TAK [1]. But because Meshtastic sends telemetry anyway there is less and less airtime for chat messages, and it gets to the point where you can't talk to people. For small groups this is fine, for bigger groups/meshes this is no bueno.
[1] https://tak.gov/ - (A) stands for Android app.