A person who interferes or meddles in the affairs of others: (Dictionary.com)
A person who intrudes into a region, field, or trade without a proper license. (Dictionary.com)
Someone who becomes involved in an activity or a social group without being asked, or enters a place without permission. (Cambridge Dictionary)
An illegal or unlicensed trader. (Merriam – Webster)
One that intrudes in a place or sphere of activity. (Merriam – Webster)
If you describe someone as an interloper, you mean that they have come into a situation or a place where they are not wanted or do not belong. (Collins Dictionary)
An Intergalactic Interloper.