他的出現

'sh run | include usernames' shows the expected users (enable_15 is not listed)

'sh ver' shows that the last change made to the config was made by enable_15

enable_15 is a built-in, non-removable system account that exists as a placeholder for Privilege Level 15 access.

username enable_15 defaulty showing ASDM, but not real username!

When you type enable at the command line, it's actually getting translated to enable 15 behind the scenes.

So when you type that in, you are indirectly logging in to the enable_15 account