If it helps, I have a positive rep with Scarlett and here were the points I got
-I talked to her before battling Connor. She asked if I thought she'd be able to beat him one day and I said yes. (this also lowers rep with Connor)
-After meeting Ava about midway through Route 1, I went back to the jungle and visited Scarlett while she was training
-The battle on Route 2 which I assume always raises her rep
-I didn't do this, but in Celeste City when you first visit, you can talk to her and she asks whether you'd prefer to travel with her or Ava.
-I'm pretty sure getting the bow itself raises reputation with her, because that seems to have made the difference between one file where rep was lowered at
and another file where it was raised, with getting the bow being the only difference between the two files