Adjective to describe someone who values and nurtures their interpersonal relationships

Warm-hearted adjective: warm-hearted; adjective: warmhearted sympathetic and kind. "a warm-hearted, affectionate girl" synonyms: kind, warm, kind-hearted, kindly, soft-hearted, good-hearted, big-hearted, tender-hearted, tender, loving, caring, feeling, unselfish, selfless, benevolent, humane, good-natured, affectionate; More gentle, mild, indulgent, friendly, open, sympathetic, understanding, compassionate, charitable, generous, magnanimous "they are perhaps the most generous, warm-hearted people in the universe" antonyms: unkind, unfriendly

Also

Caring adjective If someone is caring, they are affectionate, helpful, and sympathetic. He is a lovely boy, very gentle and caring. ...a loving, caring husband.

A combination of the two is "Warm and caring"

People who are introverted, and who have fewer friends, can still be fiercely loyal to the friends they do have, valuing and nurturing them. So, it's not really about quantity (as garrulousness might imply) but quality.

Therefore, I think a good adjective here is committed:

having made a pledge or commitment to someone (such as a romantic partner) or something (such as a cause) · committed partners · a committed parent/teacher strongly committed to the fight for equal rights ; also : characterized by such a pledge or commitment · two people in a committed relationship

Sociable:

inclined by nature to companionship with others of the same species.

marked by or conducive to friendliness or pleasant social relations.

(M-W)

Also

Extrovert:

an energetic happy person who enjoys being with other people: Most sales people are extroverts.

(Cambridge Dictionary)