system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo] Why was Cortana designed to resemble a naked lady? How did Sgt. Johnson survive the end of Halo CE?

Cortana is just an AI, so what’s the point of giving her a naked body, let alone a body? I know she’s based off of Dr. Halsey, but the two look quite different.
gpt
Cotana, like all smart AIs, chose her own Avatar during her creation. She looks the way she does because that's what she decided she wanted to look like, what she sees herself as. Her self image. 

Once chosen a smart AI's Avatar can't be changed. Attempting to do so results in malfunctions, glitches, and even total crashes of the AI. Nobody's yet figured out why or how this happens. The AI can change minor details about their Avatar, though. So Cortana can -and did- change her hair style, colour, form of dress, etc but no matter what she will always look like a young woman who resembles a younger Doctor Halsey.

As to why AIs have Avatars to begin with, that's mainly for human interaction. Many people would feel uncomfortable interacting with essentially an omnipresent, disembodied voice that has the power to control all the technology around you. Giving them an Avatar gives people something to focus on. Eyes (in most cases) to look at when talking, facial expressions (in most cases) that they can read. An extreme form of a user friendly interface, basically.