To get this achievement you need to get all three bars filled up for each character. This requires you to serve them the correct drinks. When you learn a characters name they will appear on your phone. Press X and select Tomodachill to see your progress. You need three blue bars on all 11 characters.
I highly suggest playing ENDLESS MODE and getting all the special drinks first. Check out my guide on Master Brewer
to get all the drink combos. This will take away most of the guess work.
It's pretty simple to follow their request. There's only three that I would pay attention to.
1. Serve Gala the Gala Had drink when he request a secret combo. Especially on Day 11: Tea/milk/ginger
2. Serve Neil Milky Way when he wants you to "Surprise him"Day 8: milk/honey/mint
3. Serve Freya the Bedchamber on Day 11: milk/cinnamon/honey. She WANTS an espresso, but NEEDS a Bedchamber. It's the only time you don't serve what the customer request.