It belongs to Jekyll. Mr. Guest and Utterson are fictional characters. Mr. Guest is Utterson's clerk, as well as the person Utterson confides in. Along with being trustworthy, Mr. Guest is very skillful. One of his main skills is his handwriting capabilities.
This skill comes to use when Utterson asks Mr. Guest to examine Hyde's handwriting. After his observation, he identifies that though Hyde's and Jekyll's handwriting are similar, Hyde's is more slanted. These characters are from the book The Strange Case of Dr. Jekyll and Mr. Hyde, which is a book by Robert Louis Stevenson.
According to Mr. Guest, the letter seems to have come from Hyde. The story of Dr. Jekyll and Mr. Hyde has been around for a long time but even kids are introduced to this curious story probably because it is different from what people are used to. Dr. Henry Jekyll is the friend of Mr. Utterson and he would like to know why there is something strange that surrounds his friend and the evil Dr. Hyde.
This book was published in the late 1800s but it has been reprinted several times. Right now, it is considered a classic and there are some people who become exposed to it at an early age and actually like it.