Positive:
* Leales: Loyal
* Cariñosos: Loving, affectionate
* Divertidos: Fun, entertaining
* Apoyo: Supportive
* Confiables: Trustworthy
* Generosos: Generous
* Comprensivos: Understanding
* Paciente: Patient
* Simpáticos: Likeable, friendly
* Amigables: Friendly
* Atentos: Attentive
* Honestos: Honest
* Estables: Stable
* Positivos: Positive
Negative:
* Falsos: Fake
* Inconstantes: Inconsistent
* Egoístas: Selfish
* Traicioneros: Treacherous
* Desleales: Disloyal
* Negativos: Negative
* Toxicos: Toxic
Neutral:
* Cercanos: Close
* Antiguos: Old (as in longtime friends)
* Nuevos: New (as in recent friends)
* Casuales: Casual
* De confianza: Trustworthy
The best adjective to use will depend on the specific context and the kind of friends you are describing.