If it's not a pixel art game where you can have perfect circles, use a spot light. Since it's 2D, just have it pointing down the Z axis. If you'd like, you can also add a [Cookie texture][1] to add noise or a special shape to the light.
In case it IS a pixel art game, you could still do the above, except you'd need an advanced shader of sorts to make the light create a pixelated area. I don't go near shaders so I'm not even all that sure it's possible.
Let me know if you need more help, or a more in-depth explanation.
ps. some quick tips about Unity Answers from a 2+ year user:
1) If you want to make things more clear, edit your question, don't add a comment to it.
2) Don't be desperate. If someone knows the answer, they would have posted it already. Saying "please help" won't make people magically know the answer to your question.
3) include as much info in your question. In this case, visual style of the game would be helpful to know what types of things work or don't work.
These tips will make it easier for you to get help in Unity Answers.
[1]: https://docs.unity3d.com/Manual/Cookies.html
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