Explaination,

In most trios that i've seen in media (Jujutsu Kaisen, Naruto, Demon Slayer etc.) they have these traits:

The Main Character-

Self-explanatory, its the leader of the group and is determined to achieve their goal whatever it is. They are also usually pretty goofy.

The Main Character's Best Friend/Rival-

They are there to build up the main character and usually has a backstory and gets the main character into whatever theyre doing.

The Girl-

Also pretty self-explanatory. The girl in the group. She's either girlboss or a weak person. This can vary with each trio so you can also say its the most "feminine" or weakest link.

Frisk has main character syndrome, Anomaly is the best friend, and Chara is the girl. Come back once Chapter 1 is finished and you'll see what I mean ahah-