The present disclosure is for an endoscope assembly comprising an endoscope and a disposable sheath cover. The endoscope comprises an illumination window adjacent to an observation window situated along a lateral side of a distal end portion, and an elongated slot to accommodate biopsy instruments. The sheath further comprises an end cap fitted around the end portion of the endoscope with a capsule secured to its inner wall. When the end cap engages the endoscope, the capsule is ruptured to release a transparent liquid immersing the illumination window and the observation window. A soft connector is bonded to the end cap covering the elongated slot, and to an outer surface of a biopsy channel. The soft connector further comprises an opening allowing the biopsy channel to pass through. The sheath can accommodate air/water tubes, and is secured onto the shaft of the endoscope via a cuff.