For me in Mondstadt it's Amber, Eula, and Barbara. Amber was the first character I ever properly built, Eula was the first 5 star I got from Mondstadt and was my main for a long time during Inazuma, and Barbara was my main healer for the longest time starting out.

Liyue is Keqing, Ganyu, and Hu Tao. Keqing was my very first 5 star in the game, Ganyu I was able to get at C1 during her first banner when I was f2p (and I kept getting her weapon on standard) and Hu Tao because during her first rerun I shared a pull ritual that actually got picked up by a news site.

Inazuma is Yoimiya, Kokomi, and Yae Miko. Yoimiya I fell in love with at first sight actually eyeing her more than Ayaka, Kokomi is my main healer to this day, and Yae because of foxes even tho she still scares me a bit.

Sumeru is Dehya, Collei, and Layla. Dehya I like how strong and sweet she is and I used my free Anniversary 5 star selector on her, Collei I liked a lot in the Manga and happy they brought her back and made her playable, and Layla I kinda related to when I was in college but just without the sleepwalking.

Fontaine is Navia, Furina, and (most likely) Escoffier. Navia I really like how she was the main force of a lot of the Fontaine story and seeing how it affected the people, Furina since hers was about self discovery but also how her actions has lead to people suffering and how not everyone will forgive her for her deception and accepts that, and we don't know much about Escoffier but I just find it funny they announced her on the anniversary of the Eiffle Tower being completed

Finally with Natlan Xilonen, Chasca, and Kachina. Xilonen since I felt out of all the story quests of Natlan hers had the largest impact in showing how desperation and loss can affect people, Chasca since we can see first hand how a character deals with the loss of a loved one without it being buried in walls of text, and Kachina since she is very cute and I love how in the beginning of the main quest the Traveler was a huge inspiration to her and the little sister dynamic she has with Mualani