You meet Tritera inside

Inside the inn you indeed find Tritera talking with the innkeeper but the conversation was obviously casual. As you approach she says goodbye and leads you back to the Mirror room.

Jacub : "wait, am I going to be able to walk through the mirror?"

Tritera : "Not without my help, mirror magic is surprisingly advanced. But as long as you hold on to the staff you'll be in resonance with the spell."

As you get to the mirror Tritera raises her staff and taps it gently on the glass 5 times in a precise and practiced pattern.

Tritera : "Alright, the spell is going, first time people always hesitate and it's awkward waiting for you if I'm on the other side so you're going first"

You step up to the mirror, it looks awfully solid to you. Formally handing your dad half the bag of silver and the small mirror was probably the point of no return but stepping into a mirror that you lacked any means to return through certainly had a finality to it. You think once more of the town, of home, of your family, all that you're leaving behind. It's not a long list, you would miss very little of this place. You put your hand that isn't holding onto the staff through the portal.

It feels like nothing. like your hand is in another room, maybe slightly cooler than the Inn, and in fact you can see your hand in the mirror that is now once again clearly showing the guild mirror room. Next you brave a foot, steadying yourself on the other side. The mirror was at an angle and this did very strange things to your balance. It felt like your foot needed to go further than it should to find purchase on the stone floor. Next the rest of your arm up to your shoulder. You were now halfway into the mirror but you are having trouble convincing your head to make the transition.

Tritera : "Alright get on with it"

She gently but with deliberation pushes her end of the staff forward until you finally tumble through entirely to the other side.