Sarah Nicole Hammer
Sarah Hammer was a childhood gal pal of Christian from age 2 until Christian moved away when they were both approximately ten years old. By 2005 Chris had come to regard as his best and closest friend, although it is unclear whether they had much, if any contact beyond childhood.
Childhood
Chris continues to cherish many important memories with Sarah, such as riding swing sets and playing hide and seek. Once Sarah tried to teach Chris to roller-skate, but Chris fell down and gave up.
Chris and Sarah were the same age and may have attended the same elementary school, but Chris moved to Richmond to attend middle school. For most normal people this would probably be the end of the relationship.
Chris has suggested that he did not originally view Sarah as a potential sweetheart; this decision apparently came later.
Wes Iseli
By Chris's admission, he has not had much contact with Sarah since he moved to Richmond, although at some point he apparently concluded that they were destined to be lovers. The revelation that Sarah was dating local magician/heart-throb Wes Iseli was clearly traumatic for Chris. On one occasion Chris saw Sarah sitting in Wes's lap, which made Chris visibly jealous; this incident was important enough (to Chris, anyway) to document in his comics.
Chris regards losing Sarah to Wes to be the beginning of his Love Quest. Although this event technically changed nothing--Chris had never had a girlfriend and still did not--it provoked him into action, giving him a reason to actively seek out and obtain a Boyfriend-free girl.
The lyrics of Christian's song BFF's (Best Friends Forever) seem to be addressing Sarah, with Chris conceding the impossibility of romantic love and seeking greater fulfillment in a closer platonic friendship. His comparison of "your boyfriend" (presumably Wes) to Black Sonichu recast himself and the couple into the hero-lover-rival troika that he developed for Sonichu, and then further refined by featuring Wes as an evil hedgehog in his own right. Thankfully, Wes's career proved resilient enough to survive the trauma.
William Spicer
At some point Chris learned that Sarah broke up with Wes and then subsequently met another man, William Spicer. The ups and downs of this news are captured in how the fictional Chris takes the news in Sonichu #2. Christian does not seem to know anything about Spicer, to the point that when depicting Sarah and William's wedding he depicts the groom as a Yu-Gi-Oh! character so he won't have to draw the actual person. Chris most likely did not attend the wedding, and it is possible that he is only assuming that Sarah married Spicer.
Sarah in the Comics
Main article: Saramah Rosechu
Most, if not all of Sarah's appearances in Chris's comics are in the form of Saramah Rosechu, using the powers bestowed upon her by the Anchuent Prophecy to transform into a furry. She has no actual personality beyond asserting her deep, unabiding friendship with Christian, which again suggests that he knows nothing about this person except what he likes to think she feels about him.