what is the "messages agent" i am asked to sign into every time i turn on my Mac?

Back up all data.

Launch the Keychain Access application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Keychain Access in the icon grid.

Select the login keychain from the list on the left side of the Keychain Access window. If your default keychain has a different name, select that.

If the lock icon in the top left corner of the window shows that the keychain is locked, click to unlock it. You'll be prompted for the keychain password, which is the same as your login password, unless you've changed it.

Right-click or control-click the login entry in the list. From the menu that pops up, select Change Settings for Keychain "login". In the sheet that opens, uncheck both boxes, if not already unchecked.

From the menu bar, select

Keychain Access ▹ Preferences ▹ First Aid

If the box marked Keep login keychain unlocked is not checked, check it.

Select

Keychain Access ▹ Keychain First Aid

from the menu bar and repair the keychain. Quit Keychain Access.

I have this problem too.

I don't want to keep my login keychain unlocked all the time, for security reasons.

The "Messages Agent" and other agents that pop up with requests for the login keychain are applications that I don't use, and I don't want them using the key chain just because I started my computer. How do I disable these agents individually from asking for the keychain in the background?

Are there no answers to this but to keep the keychain open all the time.

Bet the NSA loves that, and Apple can disavow any responsibility.