Should you reintroduce characters in second book in series?

Hey all!

I finished writing and published "The Shadows" earlier this year. I am working on the second book in the series currently. I'm unsure if I should reintroduce the characters?

The second book will have some elements from the first book, but ideally, it can be read as a stand alone. So you COULD read it as a series or you could read each book individually and still have it make sense. Would you briefly reintroduce/give a little back story on each of the characters or do you think it would get too repetitive for those reading it as a series?