das Wesen
nature
The inner character, fundamental nature, or true essence of a person, thing, or idea.
Trotz seiner rauen Schale hat er ein freundliches Wesen.
Despite his rough exterior, he has a friendly nature.

essence / being
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nature
The inner character, fundamental nature, or true essence of a person, thing, or idea.
Trotz seiner rauen Schale hat er ein freundliches Wesen.
Despite his rough exterior, he has a friendly nature.

'Das Wesen' is closely related to the verb 'sein' (to be) (past participle: gewesen). It describes the essence, the way of being, or the existence of something or someone.
Das Wesen der Freundschaft ist Vertrauen.
The essence of friendship is trust.
You distinguish the meanings 'character' and 'living being' from the context. 'Das Wesen' as character describes a person's personality. 'Das Wesen' as a living being refers to a creature and is often used in the plural or in fantasy and science-fiction contexts.
Ihr ruhiges Wesen macht sie zu einer angenehmen Kollegin.
Her calm nature makes her a pleasant colleague.
To describe a person's character, 'Wesen' is often combined with an adjective. Common examples are 'ein freundliches Wesen' (a friendly nature), 'ein sanftes Wesen' (a gentle nature), or 'ein fröhliches Wesen haben' (to have a cheerful nature).
Der Hund hat ein sehr verspieltes Wesen.
The dog has a very playful nature.
In the meaning of 'system' or 'sector', 'Wesen' appears almost only as the second part of compound nouns (e.g. as '-wesen'). It then describes the entirety of all structures in a field.
Das deutsche Gesundheitswesen steht vor großen Herausforderungen.
The German healthcare system faces major challenges.
human / person
'der Mensch' and 'das Wesen' can both talk about living persons. But 'Mensch' clearly means a person or a human being as a species. 'Wesen' is broader: it can be a human, an animal, or even an imaginary figure. In addition, 'Wesen' often means the inner character of a person. Learners may confuse the words because both can sometimes mean a person. For an ordinary person in everyday life, 'Mensch' sounds more natural. 'Wesen' fits better when you are talking about the type, character, or a special creature.

In dem Märchen lebt ein kleines Wesen tief im Wald.
In the fairy tale, a small creature lives deep in the forest.
Meine Schwester ist ein sehr hilfsbereiter Mensch.
My sister is a very helpful person.