I forgot how I disabled the co-pilot in the game which was several years ago. Now I notice that in the cockpit view of a Cessna, he's not there but in the external view, the co-pilot is visible. I did a search for that but mostly the answers were related to MSFS 2024. 

I think ....

set his/her weight to zero

35 minutes ago, Twenty6 said:

set his/her weight to zero

Do you remember where do you do that in the settings?

Easy. FLY the aircraft 😜

12 hours ago, Rodster said:

Do you remember where do you do that in the settings?

MS Copilot says: 

To set the copilot weight to zero in MSFS 2020, go to the weight and balance menu and adjust the copilot's weight slider to zero before closing the menu. Make sure to do this while the aircraft is in a "cold and dark" state for it to save properly.