- "Let's begin the operation!"
- — Said before starting an operation.
Dr. Derek Stiles is a genius but rather bumbling surgeon that works for Caduceus. Gifted with the powers of the Healing Touch, he works on the forefront against GUILT. Originally working for Hope Hospital under the supervision of Dr. Hoffman, he was transferred to Caduceus after saving a victim from a strain of GUILT.
Derek grew up in Northridge, a suburb just outside Angeles Bay. After losing his father to an incurable disease, this starts his resolve to become a doctor.
After completing his residency at the beginning of Under the Knife/Second Opinion, he was eager to start operating as a full time surgeon. While he was, at first, slightly irresponsible, he eventually pulled his act together in order to save a car accident victim while no other doctors were available.
Derek soon was discovered to possess the Healing Touch. While Robert Hoffman discouraged him from using it, Derek practises until he can invoke it at will. It is helpful to note that his form of this skill is linked to Asclepius, the Greek god of medicine. He would tense his index finger and imagine a star, then focus on it to gain an augmented state of concentration.
After a successful operation, Derek was noted for his abilities and was offered a job at Caduceus USA. Initially preferring to continue work at Hope Hospital, he eventually understood the threat of GUILT and joined Caduceus, eventually working to become the man to end the threat of GUILT.
Derek makes a few cameos in Trauma Team, though his face never seen. He is the one who has diagnosed Naomi Kimishima with the genetic disease she was afflicted with, and gave instructions, via phone call, to CR-S01 on how to treat Twisted Rosalia when Naomi became infected with it.
Under the Knife 2
Ever since ending the threat posed by GUILT three years ago, Derek has become a world-renowned surgeon. He is dedicated to his employer, Caduceus, and his job: saving patients. During surgery he is calm and collected- not one to crack under pressure. However, during this title he had problems with being overconfident and taking pride in his Healing Touch. After relying on it in a certain operation and failing, he realized how conceited he had been.
Trauma Center: New Blood
Derek and Angie both appear in Chapter 7-4 of New Blood as representatives of Caduceus Japan, providing information on Stigma. Additionally, they are seen in the side operation, Challenge 4. In it, he tests Markus and Valerie using a simulation made by Caduceus's research director (assumed to be Victor) that contains Cheir and Soma, as well as some of the original GUILT, namely Immature Kyriaki, Mature Kyriaki, Deftera, and Immature Savato.
Derek is a young man, with messy light-brown hair and with matching light brown eyes with square-rimmed glasses. He commonly sports brown shoes when working.
During his time within Hope Hospital, he wears a normal green-blue scrubs uniform with his name-tag upfront with pens hanging in his pocket.
During his time within Caduceus, he wears the normal Caduceus uniform with a belt holding his medical tools. He wears his magnifying goggles around his neck, which are connected to a remote on his belt.
In his casual attire, he wears a tan coat over a black and white polo shirt tucked-in his blue pants.
In Under the Knife 2, he now wears a white lab coat with his Caduceus ID card over a white scrubs top with black pants. He hangs a stethoscope around his neck alongside his identification card. He also wears white rubber gloves.
In New Blood, he is now older, sporting a small beard. He wears a tan coat over a white dress-shirt with a plaid tie.
Derek is a hard-working surgeon that is devoted to saving the lives of his patients. Although irresponsible and doubtful at times, it does not stop him from doing his job.
In Under the Knife/ Second Opinion, Derek initially shows a lack of focus and attention which almost caused one of his patients to die. Following the incident, Angie yells at him and questions him becoming a doctor, so he believes he is useless as one. While off duty, he hears about a car crash and realizes that nobody at Hope Hospital would be able to save the victims in time. Following a single-handed rescue operation, Derek becomes more confident.
Throughout the series, it is hinted that Derek has a severe problem with solving puzzles.
It has also been seen that Derek is a bit of a flirt when it comes to older women, to the point where he's been called "very smooth". Unfortunately, when he is caught off guard and asked about who he truly loves, he becomes embarrassed and shy.
- "I won't let you take Dr. Hoffman!" — Trauma Center: Second Opinion; Vulnerability
- "I won't let you die!" — Operation Diagnose quote
- "I won't let Professor Blackwell die!" — Death Awaits All
- Derek Stiles' initials are "DS", which likely references the Nintendo DS platform the early Trauma Center games are played on.
- Derek is the first playable surgeon that the player has the opportunity to operate on.
- Derek is one of two patients that become infected with two types of GUILT at the same time, Paraskevi and Kyriaki.
- Derek made a cameo in Trauma Team, yet he was shown only from the back. His old friend, Gabriel Cunningham, consulted him about the mutated Rosalia Virus colony inside Naomi Kimishima's heart, and he advised CR-S01 on how to remove the colony. His voice is also heard in Naomi's flashbacks, where he informs her that she has a genetic disease and will likely die within a few months' time.
- A poster with Angie in a bikini can be barely seen on Derek's bulletin board in his apartment.
- Derek apparently has a crush on Angie Thompson, though this is never directly stated. It is heavily implied, however, as he considers her his "loved one" at the end of Under the Knife 2.
- Derek along with Naomi are the only protagonists to appear in multiple games.