He was a sleazy businessman working for the Nakatomi Corporation. A cocaine addict and stereotypical yuppie, Ellis arrogantly attempted to assist Hans Gruber in betraying John McClane to save his own life, only to get himself killed in the process.
Ellis was first seen flirting with the married Holly, asking her to come over to his place for Christmas dinner. Although Holly declined because she has children, Ellis persisted. When John McClane arrives, Ellis was hiding in Holly's office snorting cocaine. He claims to John and his boss Joe Takagi that he was in her office to "making a call" because that was "the nearest phone". McClane realized what Ellis was doing and remarked that he "missed some". Because Ellis was bragging how he gave Holly a nice rolex watch as a gift for her big sale, McClane realized that he was putting the moves on his wife, making him dislike Ellis strongly.
Helping Hans Gruber
When the terrorists took over the Nakatomi Plaza, he panicked, whereas Holly and Takagi were calm. Although the terrorists were not hurting the hostages in any way, Ellis was too concerned with his own life to let McClane jeopardize it. He promptly snorted some cocaine and insultingly asked one of the terrorists if he "sprechen ze talk". Ellis was brought into Holly's office to speak with Hans Gruber, as he wished to use his charm and lying skills to barter with him. Ellis claimed to Hans that he had known McClane for years when he had only first met him earlier that evening. Ellis told Hans who the uninvited guest was and Hans called McClane on the radio, giving his identity away to the police, who then realized that John was not one of the terrorists. Hans said that 'a very special friend' wishes to talk to him, and John almost panics, worried that the terrorists have Holly. However, much to his relief and confusion, Ellis answers instead, addressing him as 'John Boy'. Ellis tries to convince McClane to give up the detonators that, unbeknownst to Ellis, Hans needs to blow the roof and kill the hostages. Ellis says that McClane should give himself up so the terrorists can negotiate with the LAPD and everyone can leave. Luckily, McClane knows that if he agrees then they will both get killed, and he refuses, claiming that Harry does not know what kind of people he is dealing with and that he is going to get himself killed. Ellis then asked why Hans was picking up his pistol, because McClane can't see it. Ellis gave the walkie-talkie back to Hans. Ellis tried to talk to McClane again, but he wouldn't answer and Hans promptly shot Ellis in the forehead with his chrome Heckler & Koch PM713 in order to make McClane realize that he wasn't playing games anymore. His body was dragged out of the office where the rest of the hostages saw his corpse being dragged away, terrifying them even more. His death was avenged along with Takagi by John McClane
Harry Ellis enjoyed the last moments of his life, drinking Coca Cola that Franco had given him.