Short Answer 
Cristina is often seen as sober and earnest, which can mislead others about her true character. Despite her serious demeanor, she is best described as cheerful, reflecting her genuine positivity and good humor, unlike other descriptors that fail to capture her vibrant personality.
Step 1: Understand Cristina’s Demeanor
Cristina is often perceived as sober and earnest based on her serious appearance. This initial impression can lead people to associate her with more subdued traits. However, it’s important to recognize that demeanor can be misleading and does not always reflect one’s true character.
Step 2: Identify the Right Descriptor
The word cheerful is the most fitting choice to describe Cristina as it captures her genuine positivity. It suggests that, regardless of her solemn exterior, she encounters her experiences with good humor. This quality adds depth to her personality, showing that outward appearances do not define one’s inner joy.
Step 3: Contrast Other Options
When comparing “cheerful” to other options like “comforting” and “tranquil,” it’s clear that these do not encompass Cristina’s vibrancy. While they imply a sense of calm, they lack the exuberance and humor that are central to Cristina’s character. Therefore, “cheerful” remains the most accurate description of her outlook.