Dr. Walter Baxter is a minor villain from the Alex Rider book series. Specifically, he originates from the second book Point Blanc, and serves as the surgeon for Dr. Hugo Grief's operations.
Baxter is described as being "plump, about 40 years old, with a small moustache, wire-framed spectacles and slicked yellow hair, speaking with an oily voice". He first appears as the head of the three surgeons based underneath room 13 of the Hotel du Monde (the hotel is owned by the Point Blanc Academy). He drugs the Coke that Alex Rider (under the alias of Alex Friend) drinks in the hotel's restaurant. Alex is then lowered into Baxter's laboratory via an overgrown hydraulic system built into the entire floor of room 13, where his body is photographed by Baxter and his assistants. The notes, photographs and genetic material taken from Alex will later be used for developing his evil clone... or, at least, the clone of his fictional alias. The clone will become Julius Grief.
After Alex is in the Academy and manages to climb up into the restricted third floor via the fireplace, he overhears Baxter and Grief talking. Baxter asks for another $250,000 to keep him silent, before his retirement to Spain. Grief then kills him, by shooting him in the head with a silenced automatic pistol in the surgery operating theatre, the same place where his magic worked.
Alex Rider: The Mission Dossier
In the revealed files of MI6, it's revealed that Baxter was a former gifted Harley Street plastic surgery consultant who suffered from gambling debts. He was erased from the surgery register after he was discovered (presumably by MI6) to have altered the facial features of a wanted and dangerous Serbian war criminal, on the run from international law.
After this, Baxter got involved with underworld organisations, until he somehow came into contact with Dr. Grief, possibly due to Eva Stellenbosch. He then altered the bodies of 16 children, to resemble a son from a rich and powerful family, to the point where they would have been practically impossible to tell apart. MI6 also says that he would have been killed by Grief anyway, as he "was not a reliable man".