ty!! Matsu is only available as an assist through debug tools currently, I've been wanting to put her in a special place, but it might make sense to have her more easily available while everything is under construction. She can be found as a boss in a specific secret area, but can't be asked out. As far as the fish, only one suit of the fish (Ayu) are currently available (so, the top row of the fishing page).
This is actually reminding me that I should probably have a spoilered checklist of things to do for people to check to see if they've gotten everything already, because there are some secrets, usually trying out interactions to see how they feel in a larger scope.