Replacement file version.
Drag the "[CP] Nikolai" folder from my Zip into the Stardew Valley\Mods folder to overwrite the "[CP] Nikolai" folder in it.

Including Boxy and seasonal outfits.

Content Patcher version, drag the folder into Stardew Valley\Mods to install.

I would still recommend the replacement file version since the assets won't get loaded 2 times. But if the replacement file don't work for you try this version.

Portraiture version.

*I do not advise using Portraiture to install those portraits since Portraiture doesn't support seasonal outfits, so their seasonal outfits and festival portraits will not work.*
If possible, please try the replacement file instead.