The article for Milton confirms this, but SPOILERS just in case.

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The game developers confirmed that Milton is the King in one of their previous games, The Unfinished Swan. Milton basically found his way into an alternate universe using his paint brush and either doesn't want to come back or doesn't know how to get back to the real world.

SPOILERS

In their 2020 game The Unfinished Swan the MC is named Monroe. In a reddit AMA the game developers did confirm that Milton lives. Not sure if it's confirmed but many theorize the Monroe is Milton's son. This would have made Milton 19 when Monroe was born since he was born in 2011. The events of The Unfinished Swan take place in an orphanage so it's assumed something happened to him after Monroe was born.