Dr. Yabu is first introduced in episode 1 of the anime Young Black Jack getting high which can be evidentially assumed to be because of his past in vietnam as a medic.
He is shown to have PTSD and is scared of blood this is shown to have originated from seeing many of his fellow comrades wanting to die whilst covered in injuries and asking yabu to kill them as a favor. Despite his fear of blood yabu agrees to help medical student Hazama Kuroo in operation of reattaching a boys arms and legs while only staring at the heart monitor to warn Hazama of any irregularities.
He is shown to be loyal and has a strong bond with Hazama which is seen when he decides to return to the gangstas lair to help hazama. After seeing Hazama devotion and skill in the medical field despite Hazama being a student Yabu decides to return to the root of his PTSD, "Vietnam".
He later found by Hazama and shown to have overcome his fear of blood and PTSD.