Tim Marcoh (ティム・マルコー, Timu Marukō), formerly the Crystal Alchemist (結晶の錬金術師, Kesshō no Renkinjutsushi), was the leading researcher in the military's Philosopher's Stone creation project.
He is an elderly man with prominent wrinkles and a large jaw. He tends to wear lab coats though later in the series wears a coat. He has broad shoulders and a short stature (possibly due to his age). He has dark hair with white hair on the sides, demonstrating his age.
After Scar injured him, his face became slightly misshapen, and one eye half closed.
He is a kind man suffering from post-traumatic stress disorder, along with extreme guilt over the actions he committed during the Ishvalan war that resulted in the deaths of millions of Ishvalans in order to make philosopher's stones. He is an honest, hardworking man, and is not used to bluffing but was able to bluff Envy into releasing Yoki as his hostage by feigning apathy towards sacrificing him. He refuses to return to the government due to the disgust towards their actions and even attempts to shoot Major Armstrong when he believes he was sent to retrieve him. He deeply cares for others, using the Philosopher's Stone he ran away with in order to heal the people of the village he was staying in. In addition, he was forced to work with the homunculi once again due to their threat of annihilating his village. He is so repentant of his actions that when he sees Scar for the first time he begs him to kill him and even refers to him as a God at that moment. Though unforgiving of himself for his actions along with the military and the government, he is forgiving of others and very patient. In addition, he holds no grudge towards Scar for horrifically and painfully disfiguring his face, claiming it was the face he deserved. He is brave when it matters, as he stood up to Envy and helped Scar, Al and the others to stop the homunculi. He is also very intelligent, having been the main researcher in creating the Philosopher's Stone.
Dr. Marcoh is one of the oldest known alchemists of the series and as such has incredible knowledge regarding alchemy. His most notable knowledge is related to the creation of the Philosopher's Stone, knowledge given to him by the homunculi. After numerous experiments with their creation, he is also able to destroy them, an ability which proves effective when fighting Envy.
Due to his elderly state, Dr. Marcoh is not able to use alchemy in combat, but rather he proves to be skilled when it comes to supporting tasks such as healing or deciphering codes. He is also very skilled in medicine, as more than one person has asked for his assistance, although his healing abilities may be possible through the use of the Philosopher's Stone, rather than a unique ability like Alkahestry. He also possesses the unique ability to destroy Philosopher's Stones with a single transmutation, giving him the power to cripple or kill Homunculi with a single touch.
What his epithet as the "Crystal Alchemist" means is unknown, but it is likely connected to the Philosopher's Stone in some way.
Regardless of his inability to fight, his knowledge makes him a valuable ally.
After sacrificing a number of innocent Ishvalans during the Ishval Civil War to create new stones in Laboratory 5, he fled the military with some imperfect stone samples. He settles down in a small country town where he uses his stones to heal the sick under the identity of "Doctor Mauro". He is later found by the Elric brothers, and he directs them to some of the research he left behind in the National Central Library to help them in their search for the Philosopher's Stone.
When first found by the Homunculi he is kept under house arrest, forced to stay put and alive or risk the destruction of his town. He is later taken into their custody to be used in their future plans, and is kept locked up deep below Central. While there, Envy takes great pleasure in tormenting Marcoh by constantly reminding him the Homunculi would massacre his hometown if he ever escaped. After some time as a prisoner, he is found by Scar. Seeing Scar as a means to an end, he tells the Ishvalan of his involvement in the Ishval Civil War in the hopes of being killed in vengeance. Scar, instead, only disfigures his face beyond recognition as a disguise, kidnaps him, and forces him to help in bringing down the Homunculi. While on the run, he is eventually discovered by Envy, who vows to take him back to Central but not before killing hundreds in a nearby village as revenge for Marcoh's escape. Knowing this, the Crystal Alchemist attacks Envy using his hidden transmutation circle to critically damage the Homunculus' core stone.
Later, after Kimblee springs Pride from his dome prison, Marcoh takes part in a ruse to deceive the two along with Heinkel and Alphonse. Alphonse gives Marcoh the Philosopher's Stone that Heinkel gave him, which he then used to heal Heinkel's wounds while Alphonse acted as a diversion. He is last seen with Dr. Knox, telling Colonel Mustang that he will use the Philosopher's Stone to give him his eyesight back on the condition that he improves conditions for the Ishvalan people, restores their homeland, and allows himself to stay there as a doctor, to which the Colonel accepts. In the manga, he accepts and lets him help the Ishvalans.
In the first anime series, Marcoh plays a significantly lesser role. After being taken into custody by Juliet Douglas, he is confronted by Lust. She questions him about the information he gave to the Elrics and subsequently murders him. His body is implied to have been eaten by Gluttony.
He later makes a brief cameo in the penultimate episode as one of the forms Envy uses to toy with Edward.