This quote gently contrasts the fantastical nature of the characters involved with the simple, relatable desire for companionship. Magnus’ contemplation about having a pet initially seems like a normal, everyday thought, but the mention of a "sullen teenage vampire" as his current companion introduces an intriguing supernatural element that adds humor and charm. It reveals a depth in Magnus' character—showing his capacity for care despite unusual and somewhat difficult circumstances.

The closing thought of getting a cat and throwing it a birthday party conveys a desire for normalcy and affection, a tender human (or supernatural being) need to celebrate and cherish relationships, even if those relationships take unexpected forms. This suggests a longing for simplicity amidst complexity, and perhaps a lighthearted wish to inject warmth and joy into life irrespective of its shadows.

Moreover, the quote reflects themes of friendship, acceptance, and the unconventional nature of family or chosen companions. It symbolizes how we all seek connection, even if it comes with quirks or challenges. The idea of throwing a birthday party for a pet emphasizes the importance of marking love and milestones, underscoring how celebratory rituals bond us together. Overall, the passage highlights how the supernatural world coexists with human emotions, blending fantasy with heartfelt humanity — a hallmark of Cassandra Clare’s storytelling.