Heather Brooks, or "Mousy," one of the most quirky, yet lovable interns, died a hasty death when she was found electrocuted in the basement of the hospital in Season 10 Episode 2. Her death brought a highly important lesson to Shane, who was quickly going down the similar path of Christina. He learned that being a shark has its inevitable downfall.
Denny Duquette's death in Season Episode 2 Episode 27 was a challenging one for the interns, specifically Izzy. His death led to many issues within the ethics and morals of Izzy as a surgeon, but one that made her grow into a stronger person.
Harold O'Malley's death in Season 3 Episode 12 was the first of the O'Malley deaths in the hospital, offering a foreshadowing of George's fateful ending. It taught the doctors a lot about how quickly and unexpectedly life can end.
Dr. Thomas, Christina's friend doctor in Minnesota taught her a lot about loyalty and friendship, and how to slow down as a surgeon. He helped her realize that life is more than just cutting. His death in Season 9 Episode 5 really brought a pause to Christina's old ways, while also humanizing her.
Henry Burton's death in Season 8 Episode 10 came as an unbelievable shock to Teddy. He was her first love beyond Owen and provided her a safe haven from her own complicated feelings. When Christina realized she had let him die on the operating table, it grounded her abilities. It showed her that she could make mistakes as well, and wasn't entirely perfect and invincible.
Charles Percy was one of the interns from Mercy West who was an initial jerk looking to make an impression on one of his fellow interns but during his last hours on the show in Season 6 Episode 24, he let his guard down and bonded with Bailey. Albeit Bailey and he were never close, his death was a hard one for Bailey to witness, as she was unable to do anything for him.