Notebook Briefcase

I have a job interview for a position of legal professional.

I 1) do not have a briefcase 2) are not things to fill it to make it interesting.
I carry a notebook or planner that I intend to keep on my knees, and also that the keys, mints ... a portfolio that I give to the investigator ...

Is that necessary or bracelet breifcase too informal?

A kit is not necessary. If you have a leather wallet that you can tuck a copy of your curriculum vitae, and a pad, pen and business card (if applicable), should be sufficient.

The most important part of any interview is to "be yourself". Toting along a towel, if you do not do it all the time, stands out as being against nature.

Looks like you have what you already have all aligned. Good luck for the interview!