Both Uses of

principal
in
The Magic Mountain

He then spent a few years serving at one of the Society's schools as a prefect—a headmaster, a principal in charge of their young pupils.†
Chpt 6.3 *

principal = most important; or person that is most important

He had to slide out of it sideways, so that our three principal characters now stood in a triangle, the chair in the middle.†
Chpt 7.3

Definitions:

(1) (principal)
most important; or person who is most important
The exact meaning of principal can depend upon its context. For example:
- "the principal idea" or "the principal performer" -- the most important
- "the principal of the school" -- the person in charge
- "a principal of a company" -- an owner or person with controlling authority
- "the principal in a transaction" (as contrasted to the agent) -- the person on whose behalf an agent acts
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

See a comprehensive dictionary for specialized senses in finance, architecture, law, chemistry, and music.